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0706.3569
|
Andrei Lobanov
|
Compact jets as probes for sub-parsec scale regions in AGN
|
astro-ph
|
Compact relativistic jets in active galactic nuclei offer an effective tool
for investigating the physics of nuclear regions in galaxies. The emission
properties, dynamics, and evolution of jets in AGN are closely connected to the
characteristics of the central supermassive black hole, accretion disk and
broad-line region in active galaxies. Recent results from studies of the
nuclear regions in several active galaxies with prominent outflows are reviewed
in this contribution.
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No Label
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astro-ph/0108358
|
A. Lobel
|
Two Decades of Hypergiant Research
|
astro-ph
|
This article is a brief review of the research by Dr. C. de Jager and
co-workers over the past twenty years into the physics of hypergiant
atmospheres. Various important results on the microturbulence, mass-loss,
circumstellar environment, atmospheric velocity fields, and the Yellow
Evolutionary Void of these enigmatic stars are summarized. Aspects of recent
developments and future work are also communicated.
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astro-ph/0403076
|
Maxim Lyutikov
|
On the nature of eclipses in binary pulsar J0737-3039
|
astro-ph
|
We consider magnetohydrodynamical interaction between relativistic pulsar
wind and static magnetosphere in binary pulsar system PSR J0737-3039. We
construct semi-analytical model describing the form of the interface separating
the two pulsars.
An assumption of vacuum dipole spin down for Pulsar B leads to eclipse
duration ten times longer than observed. We discuss possible Pulsar B torque
modification and magnetic field estimates due to the interaction with
Pulsar A wind.
Unless the orbital inclination is $\leq 86 ^\circ$, the duration of eclipses
is typically shorter than the one implied by the size of the eclipsing region.
We propose that eclipses occur due to synchrotron absorption by mildly
relativistic particles in the shocked Pulsar A wind. The corresponding optical
depth may be high enough if Pulsar A wind density is at the upper allowed
limits. We derive jump conditions at oblique, relativistic,
magnetohydrodynamical shocks and discuss the structure of the shocked Pulsar A
wind. Finally, we speculate on a possible mechanism of orbital modulation of
Pulsar B radio emission.
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astro-ph/0402171
|
Tim Kendall
|
Discovery of very nearby ultracool dwarfs from DENIS
|
astro-ph
|
We report new spectroscopic results, obtained with UKIRT/CGS4, of a sample of
14 candidate ultracool dwarfs selected from the DENIS (Deep Near-Infrared
Survey of the Southern Sky) database. A further object, selected from the 2MASS
Second Incremental Release, was observed at a later epoch with the same
instrument. Six objects are already known in the literature; we re-derive their
properties. A further four prove to be very nearby (~10 pc) mid-to-late
L-dwarfs, three unknown hitherto, two of which are almost certainly substellar.
These findings increase the number of L-dwarfs known within ~10 pc by ~25%. The
remainder of the objects discussed here are early L or very late M-type dwarfs
lying between ~45 and 15 pc and are also new to the literature. Spectral types
have been derived by direct comparison with J-,H- and K- band spectra of known
template ultracool dwarfs given by Leggett et al.
(ftp://ftp.jach.hawaii.edu/pub/ukirt/skl/dL.spectra/) For the known objects, we
generally find agreement to within ~1 subclass with previously derived spectral
types. Distances are determined from the most recent M_J vs. spectral type
calibrations, and together with our derived proper motions yield kinematics for
most targets consistent with that expected for the disk population; for three
probable late M-dwarfs, membership of a dynamically older population is
postulated. The very nearby L-type objects discussed here are of great interest
for future studies of binarity and parallaxes.
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astro-ph/0001202
|
Pavel Kroupa
|
Stellar-Dynamics of Young Star Clusters
|
astro-ph
|
The stellar-dynamical evolution of bound star clusters during the first few
Myr is dominated by binary-binary and binary-star interactions, the rapid
sinking of the most massive stars to the centre of the clusters and mass loss
from evolving stars. The consequences of these processes for the binary and
stellar population in clusters, and for the star clusters as a whole, are
studied by following the evolution over 150 Myr of a library of compact cluster
models containing up to 10^4 stars.
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astro-ph/9803238
|
Philip R. Maloney
|
Radiation-Driven Warping. II. Non-Isothermal Disks
|
astro-ph
|
Recent work by Pringle and by Maloney, Begelman & Pringle has shown that
geometrically thin, optically thick, accretion disks are unstable to warping
driven by radiation torque from the central source. In this paper we generalize
the study of radiation-driven warping to include general power-law surface
density distributions, $\Sigma\propto R^{-\delta}$. We consider the range
$\delta=3/2$ (isothermal disks) to $\delta=-3/2$, which corresponds to a
radiation-pressure-supported disk; this spans the range of surface density
distributions likely to be found in real astrophysical disks. There is a
critical minimum size for unstable disks. The critical radius and the
steady-state precession rate depend only weakly on $\delta$. The case
$\delta=1$ divides the solutions into two qualitatively different regimes.
Nonlinear effects must be important if the warp extends to the disk inner edge
for $\delta \ge 1$, but for $\delta < 1$ nonlinearity will be important only if
the warp amplitude is large at the origin. The effects of shadowing of the
central source by the warp will thus be very different in the two regimes of
$\delta.$ In real accretion disks the outer boundary condition is likely to be
different from the zero-crossing condition that we have assumed. In accretion
disks around massive black holes in active galactic nuclei, the disk will
probably become optically thin before the outer disk boundary is reached, while
in X-ray binaries, there will be an outer disk region (outside the
circularization radius) in which the inflow velocity is zero but angular
momentum is still transported. We show that in both these cases the solutions
are similar to the zero-crossing eigenfunctions.
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0712.1243
|
D. J. Pisano
|
What are the Luminous Compact Blue Galaxies?
|
astro-ph
|
Luminous Compact Blue Galaxies (LCBGs) are common at z~1, contributing
significantly to the total star formation rate density. By z~0, they are a
factor of ten rarer. While we know that LCBGs evolve rapidly, we do not know
what drives their evolution nor into what types of galaxies they evolve. We
present the results of a single-dish HI survey of local LCBGs undertaken to
address these questions. Our results indicate that LCBGs have M(HI) and M(DYN)
consistent with low-mass spirals, but typically exhaust their gas reservoirs in
less than 2 Gyr. Overall, the properties of LCBGs are consistent with them
evolving into high-mass dwarf elliptical or dwarf irregular galaxies or
low-mass, late-type spiral galaxies.
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0708.3565
|
Istv\'an D\'ek\'any
|
The double-mode RR Lyrae variable BS Com
|
astro-ph
|
We present the frequency analysis of the multicolour time series photometry
of the field RRd variable BS Comae. The large number of data points in each of
the BV(RI)_c bands and the ~0.01 magnitude accuracy of the individual
measurements allow us a high precision analysis of the properties of the
combination frequencies due to nonlinear coupling. Through the combination of
the frequency spectra in different colors we show that except for the
components corresponding to the linear combinations of the two pulsation modes,
there are no other components present above the millimagnitude amplitude level.
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astro-ph/9807264
|
Peter Lundqvist
|
Flash ionization of the partially ionized wind of the progenitor of SN
1987A
|
astro-ph
|
The H II region created by the progenitor of SN 1987A was further heated and
ionized by the supernova flash. Prior to the flash, the temperature of the gas
was 4000 - 5000 K, and helium was neutral, while the post-flash temperature was
only slightly less than 10^5 K, with the gas being ionized to helium-like
ionization stages of C, N and O. We have followed the slow post-flash cooling
and recombination of the gas, as well as its line emission, and find that the
strongest lines are N V 1240 and O VI 1034. Both these lines are good probes
for the density of the gas, and suitable instruments to detect the lines are
STIS on HST and FUSE, respectively. Other lines which may be detectable are N
IV] 1486 and [O III] 5007, though they are expected to be substantially weaker.
The relative strength of the oxygen lines is found to be a good tracer of the
color temperature of the supernova flash. From previous observations, we put
limits on the hydrogen density, n_H, of the H II region. The early N V 1240
flux measured by IUE gives an upper limit which is n_H ~ 180 \eta^{-0.40}
cm^{-3}, where \eta is the filling factor of the gas. The recently reported
emission in [O III] 5007 at 2500 days requires n_H = (160\pm12) \eta^{-0.19}
cm^{-3}, for a supernova burst similar to that in the 500full1 model of Ensman
& Burrows (1992). For the more energetic 500full2 burst the density is n_H =
(215\pm15) \eta^{-0.19} cm^{-3}. These values are much higher than in models of
the X-ray emission from the supernova (n_H ~ 75 cm^{-3}), and it seems
plausible that the observed [O III] emission is produced primarily elsewhere
than in the H II region. We also discuss the type of progenitor consistent with
the H II region. In particular, it seems unlikely that its spectral type was
much earlier than B2 Ia.
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astro-ph/0004279
|
M. R. Schreiber
|
The response of a dwarf nova disc to real mass transfer variations
|
astro-ph
|
We present simulations of dwarf nova outbursts taking into account realistic
variations of the mass loss rate from the secondary. The mass transfer
variation has been derived from 20 years of visual monitoring and from X-ray
observations covering various accretion states of the discless cataclysmic
variable AM Herculis. We find that the outburst behaviour of a fictitious dwarf
nova with the same system parameters as AM Her is strongly influenced by these
variations of the mass loss rate. Depending on the mass loss rate, the disc
produces either long outbursts, a cycle of one long outburst followed by two
short outbursts, or only short outbursts. The course of the transfer rate
dominates the shape of the outbursts because the mass accreted during an
outburst cycle roughly equals the mass transferred from the secondary over the
outburst interval. Only for less than 10% of the simulated time, when the mass
transfer rate is nearly constant, the disc is in a quasi-stationary state
during which it periodically repeats the same cycle of outbursts. Consequently,
assuming that the secondary stars in non-magnetic CV's do not differ from those
in magnetic ones, our simulation indicates that probably all dwarf novae are
rarely in a stationary state and are constantly adjusting to the prevailing
value of the mass transfer rate from the secondary.
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astro-ph/9703082
|
Li-Xin Li
|
Effect of the Global Rotation of the Universe on the Formation of
Galaxies
|
astro-ph
|
The effect of the global rotation of the universe on the formation of
galaxies is investigated. It is found that the global rotation provides a
natural origin for the rotation of galaxies, and the morphology of the objects
formed from gravitational instability in a rotating and expanding universe
depends on the amplitude of the density fluctuation, different values of the
amplitude of the fluctuation lead to the formation of elliptical galaxies,
spiral galaxies, and walls. The global rotation gives a natural explanation of
the empirical relation between the angular momentum and mass of galaxies:
$J\propto M^{5/3}$. The present angular velocity of the universe is estimated,
which is $\sim 10^{-13} rad yr^{-1}$.
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astro-ph/9911213
|
Stuart Littlefair
|
J-band spectroscopy of cataclysmic variables
|
astro-ph
|
We present time resolved J-band (1.025 - 1.340 micron) infrared spectra of
the short-period dwarf novae (DNe) WZ Sge, VY Aqr and single spectra of the
short-period DN EF Peg and the novalike variable PX And. There is some evidence
in the spectra of VY Aqr and EF Peg that we have detected the secondary star,
both in the continuum slope and also through the possible presence of spectral
features. The spectra of WZ Sge and PX And, on the other hand, show no evidence
for the secondary star, with upper limits for its contribution to the J-band
light of 10% and 20% respectively. The spectral type of the secondary in WZ Sge
is constrained to be later than M7.5V. Using skew mapping we have been able to
derive a value for the radial velocity semi-amplitude of the secondary star in
VY Aqr of Kr = (320 +/- 70) km/s, which in conjunction with Kw from Thorstensen
& Taylor (1997) gives a mass ratio of q = 0.15 +/- 0.04.
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astro-ph/0506202
|
Sanjay K. Ghosh
|
Rotating Quark Star in Chiral Colour Dielectric Model
|
astro-ph
|
The properties of rotating quark star is studied using the equation of state
obtained from Chiral Colour Dielectric model. The results are compared with the
MIT bag model results. The frequencies in the corotating innermost circular
orbits for different central densities are evaluated and compared with the
observational results.
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No Label
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astro-ph/0611096
|
Casey Papovich
|
The Star Formation History and Stellar Assembly of High Redshift
Galaxies
|
astro-ph
|
I discuss current observational constraints on the star-formation and
stellar-assembly histories of galaxies at high redshifts. The data on massive
galaxies at z<1 implies that their stellar populations formed at z>2, and that
their morphological configuration was in place soon thereafter. Spitzer Space
Telescope 24 micron observations indicate that a substantial fraction of
massive galaxies at z ~ 1.5-3 have high IR luminosities, suggesting they are
rapidly forming stars, accreting material onto supermassive black holes, or
both. I compare how observations of these IR-active phases in the histories of
massive galaxies constrain current galaxy-formation models.
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astro-ph/0007278
|
Peter Berczik
|
Dissipative N - body code for galaxy evolution
|
astro-ph
|
The evolving galaxy is considered as a system of baryonic fragments embedded
into the static dark nonbaryonic (DH) and baryonic (BH) halo and subjected to
gravitational and viscous interactions. Although the chemical evolution of each
separate fragment is treated in the frame of one -- zone close box model with
instantaneous recycling, its star formation (SF) activity is a function of mean
local gas density and, therefore, is strongly influenced by other interacting
fragments. In spite of its simplicity this model provides a realistic
description of the process of galaxy formation and evolution over the Hubble
timescale.
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astro-ph/0002198
|
Craig J. Copi
|
Improving the Resolution of X-Ray Telescopes with Occulting Satellites
|
astro-ph
|
One of the challenges of X-ray astronomy is how to both collect large numbers
of photons yet attain high angular resolution. Because X-ray telescopes utilize
grazing optics, to collect more photons requires a larger acceptance angle
which in turn compromises the angular resolution. All X-ray telescopes thus
have angular resolution far poorer than their diffraction limit. Although
collecting more photons is a desirable goal, sometimes selective collecting
fewer photons may yield more information. Natural (such as lunar) occultations
have long been used to study sources on small angular scales. But natural
occulters are of limited utility because of their large angular velocities
relative to the telescope, and because of the serendipity of their transits. We
describe here how one can make use of an X-ray Big Occulting Steerable
Satellite (X-BOSS) to achieve very-high resolution of X-ray sources. An X-BOSS
could significantly improve the resolution of existing X-ray facilities such as
the Chandra telescope, or X-ray Multiple Mirror (XMM) satellite, and could
vastly improve the resolution of some future X-ray telescopes, particularly
Constellation X where sub-milliarcsecond resolution is possible for a wide
range of sources. Similar occulting satellites could also be deployed in
conjunction with planned space observatories for other wavebands.
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astro-ph/0601495
|
Patrice Bouchet J..
|
SN 1987A After 18 Years: Mid-Infrared GEMINI and SPITZER Observations of
the Remnant
|
astro-ph
|
We present high resolution 11.7 and 18.3um mid-IR images of SN 1987A obtained
on day 6526 with T-ReCS attached to the Gemini telescope. The 11.7um flux has
increased significantly since our last observations on day 6067. The images
clearly show that all the emission arises from the equatorial ring (ER).
Spectra obtained with Spitzer, on day 6184 with MIPS at 24um, on day 6130 with
IRAC in the 3.6-8um region, and on day 6190 with IRS in the 12-37um region show
that the emission consists of thermal emission from silicate dust that
condensed out in the red giant wind of the progenitor star. The dust
temperature is ~166K, and the emitting dust mass is ~2.6 x 10-6 Msun. Lines of
[Ne II]12.82um and [Ne III]15.56um are clearly present, as well as a weak [Si
II]34.8um line. We also detect two lines near 26um which we tentatively ascribe
to [Fe II]25.99um and [O IV]25.91um. Comparison of the Gemini 11.7um image with
X-ray images from Chandra, UV-optical images from HST, and radio synchrotron
images obtained by the ATCA show generally good correlation of the images
across all wavelengths. Because of the limited resolution of the mid-IR images
we cannot uniquely determine the location or heating mechanism of the dust
giving rise to the emission. The dust could be collisionally heated by the
X-ray emitting plasma, providing a unique diagnostic of plasma conditions.
Alternatively, the dust could be radiatively heated in the dense UV-optical
knots that are overrun by the advancing supernova blast wave. In either case
the dust-to-gas mass ratio in the circumstellar medium around the supernova is
significantly lower than that in the general ISM of the LMC, suggesting either
a low condensation efficiency in the wind of the progenitor star, or the
efficient destruction of the dust by the SN blast wave.
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astro-ph/0507107
|
Donald C. Ellison
|
Thermal Particle Injection in Nonlinear Diffusive Shock Acceleration
|
astro-ph
|
Particle acceleration in collisionless astrophysical shocks, i.e., diffusive
shock acceleration (DSA), is the most likely mechanism for producing cosmic
rays, at least below 10^{15} eV. Despite the success of this theory, several
key elements, including the injection of thermal particles, remains poorly
understood. We investigate injection in strongly nonlinear shocks by comparing
a semi-analytic model of DSA with a Monte Carlo model. These two models treat
injection quite differently and we show, for a particular set of parameters,
how these differences influence the overall acceleration efficiency and the
shape of the broad-band distribution function.
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astro-ph/0411256
|
Nate McCrady
|
Mass Segregation and the Initial Mass Function of Super Star Cluster
M82-F
|
astro-ph
|
We investigate the Initial Mass Function and mass segregation in super star
cluster M82-F with high resolution Keck/NIRSPEC echelle spectroscopy.
Cross-correlation with template supergiant spectra provides the velocity
dispersion of the cluster, enabling measurement of the kinematic (virial) mass
of the cluster when combined with sizes from NICMOS and ACS images. We find a
mass of 6.6 +/- 0.9 x 10^5 M_sun based on near-IR light and 7.0 +/- 1.2 x 10^5
M_sun based on optical light. Using PSF-fitting photometry, we derive the
cluster's light-to-mass ratio in both near-IR and optical light, and compare to
population synthesis models. The ratios are inconsistent with a normal stellar
initial mass function for the adopted age of 40 to 60 Myr, suggesting a
deficiency of low-mass stars within the volume sampled. King model light
profile fits to new HST/ACS images of M82-F, in combination with fits to
archival near-IR images, indicate mass segregation in the cluster. As a result,
the virial mass represents a lower limit on the mass of the cluster.
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astro-ph/0411363
|
Sergei A. Klioner
|
Refining the relativistic model for Gaia: cosmological effects in the
BCRS
|
astro-ph
|
This paper represents a first attempt of embedding the Barycentric Celestial
Reference System, the fundamental relativistic reference system to be used for
the modeling of Gaia observations, into some cosmological background. The
general Robertson-Walker metric is transformed into local coordinates where the
cosmological effects are represented as tidal potentials. A version of a
cosmological BCRS is then suggested to lowest order. The effects of
cosmological background on the motion of the solar system are estimated and
found to be completely negligible. The relation to the de Sitter and
Schwarzschild-de Sitter solutions is discussed.
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astro-ph/0010087
|
Nario Kuno
|
Distribution and Kinematics of Molecular Gas in Barred Spiral Galaxies.
I. NGC 3504
|
astro-ph
|
We present the results of the CO mapping observations of the barred spiral
galaxy NGC3504 with the Nobeyama 45-m telescope. The distribution of the
molecular gas shows offset ridges which correspond to the distribution of HII
regions along the bar. The velocity perpendicular to the bar decreases abruptly
at the ridge. The velocity change implies that the molecular gas changes the
direction of its motion to parallel to the bar at the ridge. Since the position
angle of the major axis of the bar and the line of nodes are almost the same in
NGC 3504, an upper limit to the pattern speed of the bar can be derived
directly from the radial velocity. The resultant upper limit is 41 km/s/kpc
which is much smaller than that derived with an assumption that the corotation
radius is located at the end of the bar (77 km/s/kpc). The corotaion radius
derived from our upper limit is more than two times larger than the length of
the semi-major axis of the bar in NGC 3504.
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0811.3216
|
Anze Slosar
|
Alignment of galaxy spins in the vicinity of voids
|
astro-ph
|
We provide limits on the alignment of galaxy orientations with the direction
to the void center for galaxies lying near the edges of voids. We locate
spherical voids in volume limited samples of galaxies from the Sloan Digital
Sky Survey using the HB inspired void finder and investigate the orientation of
(color selected) spiral galaxies that are nearly edge-on or face-on. In
contrast with previous literature, we find no statistical evidence for
departure from random orientations. Expressed in terms of the parameter c,
introduced by Lee & Pen to describe the strength of such an alignment, we find
that c<0.11(0.13) at 95% (99.7%) confidence limit within a context of a toy
model that assumes a perfectly spherical voids with sharp boundaries.
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0706.0010
|
Fan Yizhong
|
A Canonical High Energy Afterglow Emission Light Curve?
|
astro-ph
|
We present self consistent calculations of Synchrotron self Compton (SSC)
radiation that takes place within the afterglow blast wave and External inverse
Compton (EIC) radiation that takes place when flare photons (produced by an
internal process) pass through the blast wave. We show that if our current
interpretations of the Swift XRT data are correct, there should be a canonical
high energy afterglow emission light curve. We expect that GRBs with a long
term X-ray flattening or X-ray flares should show similar high energy features.
The EIC emission, however, is long lasting and weak and might be outshined by
the SSC emission of the forward shock. The high energy emission could be well
detected by the soon to be launched GLAST satellite. Its detection could shed
new light on the conditions within the emitting regions of GRBs.
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astro-ph/0312130
|
Raghvendra Sahai
|
X-Ray Emission from the Pre-Planetary Nebula Henize 3-1475
|
astro-ph
|
We report the first detection of X-ray emission in a pre-planetary nebula,
Hen 3-1475. Pre-planetary nebulae are rare objects in the short transition
stage between the Asymptotic Giant Branch and planetary nebula evolutionary
phases, and Hen 3-1475, characterised by a remarkable S-shaped chain of optical
knots, is one of the most noteworthy members of this class. Observations with
the Advanced CCD Imaging Spectrometer (ACIS) onboard the Chandra X-Ray
observatory show the presence of compact emission coincident with the brightest
optical knot in this bipolar object, which is displaced from the central star
by 2.7 arcsec along the polar axis. Model fits to the X-ray spectrum indicate
an X-ray temperature and luminosity, respectively, of (4.3-5.7) 10^6 K and
(4+/-1.4) 10^{31} (D/5 kpc)^2 erg s^{-1}, respectively. Our 3-sigma upper limit
on the luminosity of compact X-ray emission from the central star in Hen 3-1475
is ~5 10^{31} (D/5 kpc)^2 erg s^{-1}. The detection of X-rays in Hen 3-1475 is
consistent with models in which fast collimated post-AGB outflows are crucial
to the shaping of planetary nebulae; we discuss such models in the context of
our observations.
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astro-ph/9504057
|
Wayne Hu
|
The Physics of Microwave Background Anisotropies
|
astro-ph
|
Cosmic microwave background anisotropies provide a vast amount of information
on both structure formation in the universe and the background dynamics and
geometry. The full physical content and detailed structure of anisotropies can
be understood in a simple and intuitive fashion through a systematic
investigation of the individual mechanisms for anisotropy formation, based on
elementary gravitational and fluid dynamics.
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astro-ph/0405149
|
Shunichiro Kinoshita
|
Acoustic causality in relativistic shells
|
astro-ph
|
The motion of sound waves propagating in the perfect fluid with inhomogeneous
background flow is effectively described as a massless scalar field on a curved
space-time. This effective geometry is characterized by the acoustic metric,
which depends on the background flow, and null geodesics on the geometry
express the acoustic causal structure. Therefore by the effective geometry we
can easily study the causality on the flows. In this paper, we consider a
spherically symmetric, relativistic outflow and present the maximal causally
connected region for a super-sonic flow. When Lorentz factor of the radial
velocity of the flow is constant or obeys power-law with respect to the radial
coordinate r, we can solve it analytically. As a result we show that in the
constant case the maximum angle is proportional to inverse of Lorentz factor
and logarithmically increases with respect to r, in contrast, accelerative
expansions in power-law case make this angle bounded.
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astro-ph/0305349
|
Ivan Hubeny
|
A Possible Bifurcation in Atmospheres of Strongly Irradiated Stars and
Planets
|
astro-ph
|
We show that under certain circumstances the differences between the
absorption mean and Planck mean opacities can lead to multiple solutions for an
LTE atmospheric structure. Since the absorption and Planck mean opacities are
not expected to differ significantly in the usual case of radiative
equilibrium, non-irradiated atmospheres, the most interesting situations where
the effect may play a role are strongly irradiated stars and planets, and also
possibly structures where there is a significant deposition of mechanical
energy, such as stellar chromospheres and accretion disks. We have presented an
illustrative example of a strongly irradiated giant planet where the
bifurcation effect is predicted to occur for a certain range of distances from
the star.
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astro-ph/9411072
|
Masashi Chiba
|
Protogalactic evolution and magnetic fields
|
astro-ph
|
We show that the relatively strong magnetic fields ($\ge 1 \mu$G) in high
redshift objects can be explained by the combined action of an evolving
protogalactic fluctuation and electrodynamic processes providing the magnetic
seed fields. Three different seed field mechanisms are reviewed and
incorporated into a spherical "top-hat" model and tidal torque theory for the
fate of a forming galaxy in an expanding universe. Very weak fields $10^{-19}
\sim 10^{-23}$G created in an expanding over-dense region are strongly enhanced
due to the dissipative disk formation by a factor $\sim 10^4$, and subsequently
amplified by strong non-axisymmetric flow by a factor $\sim 10^{6-10}$,
depending on the cosmological parameters and the epoch of galaxy formation. The
resulting field strength at $z \sim 0.395$ can be of the order of a few $\mu$G
and be close to this value at $z \sim 2$.
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No Label
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No Label
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astro-ph/0012015
|
Frank Thim
|
Cepheid Calibration of the Peak Brightness of SNe Ia. X. SN 1991T in NGC
4527
|
astro-ph
|
Repeated imaging observations have been made of NGC 4527 with the Hubble
Space Telescope between April and June 1999, over an interval of 69 days.
Images were obtained on 12 epochs in the F555W band and on five epochs in the
F814W band. The galaxy hosted the type Ia supernova SN1991T, which showed
relatively unusual behavior by having both an abnormal spectrum near light
maximum, and a slower declining light curve than the proto-typical Branch
normal SNe Ia.
A total of 86 variables that are putative Cepheids have been found, with
periods ranging from 7.4 days to over 70 days. From photometry with the DoPHOT
program, the de-reddened distance modulus is determined to be (m-M)_0 = 30.67
+/- 0.12 (internal uncertainty) using a subset of the Cepheid data whose
reddening and error parameters are secure. A parallel analysis of the Cepheids
using photometry with ROMAFOT yields (m -M)_0 =30.82 +/- 0.11. The final
adopted modulus is (m -M)_0 =30.74 +/- 0.12 +/- 0.12 (d=14.1 +/- 0.8 +/- 0.8
Mpc).
The photometric data for SN1991T are used in combination with the Cepheid
distance to NGC 4527 to obtain the absolute magnitude for this supernova of
M_V^0(max) = -19.85 +/- 0.29. The relatively large uncertainty is a result of
the range in estimates of the reddening to the supernova. Thus SN1991T is seen
to be only moderately brighter (by ~ 0.3 mag) than the mean for
spectroscopically normal supernovae, although magnitude differences of up to
0.6 mag cannot be ruled out.
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No Label
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No Label
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0812.0384
|
Marina Romanova
|
Discovery of Drifting High-frequency QPOs in Global Simulations of
Magnetic Boundary Layers
|
astro-ph
|
We report on the numerical discovery of quasi-periodic oscillations (QPOs)
associated with accretion through a non-axisymmetric magnetic boundary layer in
the unstable regime, when two ordered equatorial streams form and rotate
synchronously at approximately the angular velocity of the inner disk The
streams hit the star's surface producing hot spots. Rotation of the spots leads
to high-frequency QPOs. We performed a number of simulation runs for different
magnetospheric sizes from small to tiny, and observed a definite correlation
between the inner disk radius and the QPO frequency: the frequency is higher
when the magnetosphere is smaller. In the stable regime a small magnetosphere
forms and accretion through the usual funnel streams is observed, and the
frequency of the star is expected to dominate the lightcurve. We performed
exploratory investigations of the case in which the magnetosphere becomes
negligibly small and the disk interacts with the star through an equatorial
belt. We also performed investigation of somewhat larger magnetospheres where
one or two ordered tongues may dominate over other chaotic tongues. In
application to millisecond pulsars we obtain QPO frequencies in the range of
350 Hz to 990 Hz for one spot. The frequency associated with rotation of one
spot may dominate if spots are not identical or antipodal. If the spots are
similar and antipodal then the frequencies are twice as high. We show that
variation of the accretion rate leads to drift of the QPO peak.
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No Label
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No Label
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0707.0410
|
Pawel Ciecielag
|
On gas drag in a circular binary system
|
astro-ph
|
We investigate both analytically and numerically the motion of massless
particles orbiting primary star in a close circular binary system with
particular focus on the gas drag effects. These are the first calculations with
particles ranging in size from 1m to 10km, which account for the presence of a
tidally perturbed gaseous disk. We have found numerically that the radial mass
transport by the tidal waves plays a crucial role in the orbital evolution of
particles. Numerical results are confirmed analytical calculations that do not
assume anything about origin of the radial flow in the disk. We demonstrate
that the migration rate of a particle in a disk out of radial equilibrium is
enhanced due to the enhanced mass flux of gas colliding with the particle and
the migration is always directed inward regardless of the sign of the radial
gas velocity. Within the framework of the perturbation theory we derive
general, formulae for short-term variations of the particle semi-major axis,
eccentricity and inclination in such disk. The formulae account for departures
from axial symmetry by introducing effective components of the gas velocity.
They agree with numerical results within several percent. We have also found in
numerical simulations that the tidal waves introduce coherence in periastron
longitude and eccentricity for particles on neighbouring orbits. The degree of
the coherence depends on the particle size and on the distance from the primary
star, being most prominent for particles with 10m radius. The results are
important mainly in the context of planetesimal formation and, to a lesser
degree, during the early planetesimal accretion stage.
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No Label
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No Label
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0704.0475
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Norbert Werner
|
Possible non-thermal nature of the soft-excess emission in the cluster
of galaxies Sersic 159-03
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astro-ph
|
We present an analysis of new Suzaku data and archival data from XMM-Newton
of the cluster of galaxies Sersic 159-03, which has a strong soft X-ray excess
emission component. The Suzaku observation confirms the presence of the soft
excess emission, but it does not confirm the presence of redshifted OVII lines
in the cluster. Radial profiles and 2D maps derived from XMM-Newton
observations show that the soft excess emission has a strong peak at the
position of the central cD galaxy and the maps do not show any significant
azimuthal variations. Although the soft excess emission can be fitted equally
well with both thermal and non-thermal models, its spatial distribution is
neither consistent with the models of intercluster warm-hot filaments, nor with
models of clumpy warm intracluster gas associated with infalling groups. Using
the data obtained by the XMM-Newton Reflection Grating Spectrometers we do not
confirm the presence of the warm gas in the cluster centre with the expected
properties assuming the soft excess is of thermal origin. The observed
properties of the soft excess emission are consistent with the non-thermal
interpretation. While the high density of relativistic electrons associated
with the peak of the soft emission in the cluster centre might have been
provided by an active galactic nucleus in the central cD galaxy, the underlying
population might have been accelerated in diffuse shocks.
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No Label
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No Label
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astro-ph/9609057
|
F. Haardt
|
The instrinsic UV/soft x-ray spectrum of quasars
|
astro-ph
|
The detection of a HeII absorption trough in the spectra of three
high-redshift quasars provides new constraints on the spectral shape of the UV
extragalactic background. The relative strengths of the observed flux
decrements at the rest-frame wavelengths of 1216 and 304 A require a relatively
soft radiation field at 4 ryd compared to 1 ryd. If the ionizing metagalactic
flux is dominated by the integrated light from QSOs, and the HeII reionization
of the universe was completed well before z~3, then the UV/soft X-ray spectrum
of individual quasars at these epochs must, once the cosmological ``filtering''
through material along the line of sight is taken into account, satisfy similar
constraints on the average. We model the propagation of AGN-like ionizing
radiation through the intergalactic medium using CUBA, a numerical code
developed in our prior work. We show that, in order to explain the reported
HeII absorption, any thermal component responsible for the ``soft X-ray
excess'' observed in the spectra of AGNs at z<0.3 must, at z~3, have a typical
temperature >80 eV, and a luminosity not exceeding 20% of that of the ``UV
bump''.
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No Label
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No Label
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astro-ph/0502359
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Chris Carilli
|
Radio astronomical probes of cosmic reionization and the first luminous
sources: probing the 'twilight zone'
|
astro-ph
|
The epoch of reionization (EoR) corresponds to a 'cosmic phase transition',
when the neutral intergalactic medium (IGM) becomes ionized by the first stars
and/or AGN. While the discoveries of Gunn-Peterson (GP) absorption troughs in
the spectra of the highest redshift QSOs, and large scale polarization of the
CMB, have set the first hard constraints on the EoR, the redshift and process
of reionization, and the nature of the first luminous objects, remain two of
the paramount questions in cosmic structure formation. Moreover, the GP effect
is such that observations of objects during this epoch will be difficult at
wavelengths shorter than about 1 micron. Hence, cosmic reionization, and the
formation of the first luminous objects, occurs in a 'twilight zone',
observable only at radio through near-IR wavelengths. In this talk I explore
studies of the EoR at meter through submillimeter wavelengths. I present recent
observations of the dust, molecular gas, and star formation activity in the
host galaxies of the highest redshift QSOs. These results have interesting
implications on the timescale for metal and dust enrichment, on the possibility
of co-eval formation of SMBHs and galaxies, and on the process of reionization.
I then discuss future capabilities of low frequency radio astronomy to study
the neutral IGM via the HI 21cm line, including imaging and power spectral
analyses of large scale structure in emission, and absorption studies toward
the first radio loud sources. I conclude with a summary of the VLA-VHF system
to study cosmic Stromgren spheres associated with the highest redshift SDSS
QSOs in the HI 21cm line at 190 MHz.
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No Label
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No Label
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astro-ph/0008013
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Thomas Preibisch
|
Triggered Star Formation in the Scorpius-Centaurus OB Association (Sco
OB2)
|
astro-ph
|
We explore the star formation history of the Upper Scorpius OB association,
the youngest part of Sco OB2. A wide field (160 square-degree) survey for
low-mass pre-main sequence (PMS) stars enabled us to increase the number of
known low-mass members of Upper Scorpius to nearly 100 stars. In a detailed
analysis of the locations of these stars in the HR diagram, taking proper
account of the uncertainties and the effects of unresolved binaries, we find a
mean stellar age of about 5 Myr and no evidence for a significant age
dispersion among these stars. This implies that the star formation history of
the Upper Scorpius association was dominated by a short star-burst, which
started about 5 Myr ago and ended probably not more than one or two Myr later.
Interestingly, the structure and kinematics of the HI shells surrounding the
Sco OB2 association show that the shock wave of a supernova explosion in the
nearby Upper Centaurus-Lupus association, the oldest part of Sco OB2, crossed
Upper Scorpius just about 5 Myr ago. This strongly suggests that this supernova
shock wave triggered the star-burst in Upper Scorpius.
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No Label
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No Label
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0707.1907
|
Michael P. Muno
|
A Catalog of Diffuse X-ray-Emitting Features within 20 pc of Sgr A*:
Twenty Pulsar Wind Nebulae?
|
astro-ph
|
We present a catalog of 34 diffuse features identified in X-ray images of the
Galactic center taken with the Chandra X-ray Observatory. Several of the
features have been discussed in the literature previously, including 7 that are
associated with a complex of molecular clouds that exhibits fluorescent line
emission, 4 that are superimposed on the supernova remnant Sgr A East, 2 that
are coincident with radio features that are thought to be the shell of another
supernova remnant, and one that is thought to be a pulsar wind nebula only a
few arcseconds in projection from Sgr A*. However, this leaves 20 features that
have not been reported previously. Based on the weakness of iron emission in
their spectra, we propose that most of them are non-thermal. One long, narrow
feature points toward Sgr A*, and so we propose that this feature is a jet of
synchrotron-emitting particles ejected from the supermassive black hole. For
the others, we show that their sizes (0.1-2 pc in length for D=8 kpc), X-ray
luminosities (between 10^32 and 10^34 erg/s, 2-8 keV), and spectra (power laws
with Gamma=1-3) are consistent with those of pulsar wind nebulae. Based on the
star formation rate at the Galactic center, we expect that ~20 pulsars have
formed in the last 300 kyr, and could be producing pulsar wind nebulae. Only
one of the 19 candidate pulsar wind nebulae is securely detected in an archival
radio image of the Galactic center; the remainder have upper limits
corresponding to L_R<la10^31 erg/s. These radio limits do not strongly
constrain their natures, which underscores the need for further multi-
wavelength studies of this unprecedented sample of Galactic X-ray emitting
structures.
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No Label
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No Label
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astro-ph/9906070
|
Dale Frail
|
SN1998bw: The Case for a Relativistic Shock
|
astro-ph
|
SN1998bw shot to fame by claims of association with GRB980425. Independent of
its presumed association with a GRB, this SN is unusual in its radio
properties. A simple interpretation of the unusually bright radio emission
leads us to the conclusion that there are two shocks in this SN: a slow moving
shock containing most of the ejecta and a relativistic shock (Gamma=2) which is
responsible for the radio emission. This is the first evidence for the
existence of relativistic shocks in supernovae. It is quite plausible that this
shock may produce high energy emission (at early times and by inverse Compton
scattering). As with other supernovae, we expect radio emission at much later
times powered primarily by the slow moving ejecta. This expectation has
motivated us to continue monitoring this unusual SN.
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No Label
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No Label
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0806.0371
|
Roger A. Chevalier
|
Shock Breakout Emission from a Type Ib/c Supernova: XRT 080109/SN 2008D
|
astro-ph
|
The X-ray flash 080109, associated with SN 2008D, can be attributed to the
shock breakout emission from a normal Type Ib/c supernova. If the observed
emission is interpreted as blackbody emission, the temperature and radiated
energy are close to expectations, considering that scattering dominates
absorption processes so that spectrum formation occurs deep within the
photosphere. The X-ray emission observed at ~10 days is attributed to inverse
Compton scattering of photospheric photons with relativistic electrons produced
in the interaction of the supernova with the progenitor wind. A simple model
for the optical/ultraviolet emission from shock breakout is developed and
applied to SN 1987A, SN 1999ex, SN 2008D, and SN 2006aj, all of which have
optical emission observed at t~1 day. The emission from the first three can
plausibly be attributed to shock breakout emission. The photospheric
temperature is most sensitive to the radius of the progenitor star core and the
radii in these cases are in line with expectations from stellar evolution. The
early optical/ultraviolet observations of SN 2006aj cannot be accommodated by a
shock breakout model in a straightforward way.
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No Label
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No Label
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astro-ph/9812475
|
Dr. Mark H. Finger
|
The Outbursts and Orbit of the Accreting Pulsar GS 1843-02 = 2S 1845-024
|
astro-ph
|
We present observations of a series of 10 outbursts of pulsed hard X-ray flux
from the transient 10.6 mHz accreting pulsar GS 1843-02, using the Burst and
Transient Source Experiment on the Compton Gamma Ray Observatory. These
outbursts occurred regularly every 242 days, coincident with the ephemeris of
the periodic transient GRO J1849-03 (Zhang et al. 1996), which has recently
been identified with the SAS 3 source 2S 1845-024 (Soffitta et al. 1998). Our
pulsed detection provides the first clear identification of GS 1843-02 with 2S
1845-024. We present a pulse timing analysis which shows that the 2S 1845-024
outbursts occur near the periastron passage of the neutron star's highly
eccentric (e = 0.88+-0.01) 242.18+-0.01 day period binary orbit about a high
mass (M > 7 solar masses) companion. The orbit and transient outburst pattern
strongly suggest the pulsar is in a binary system with a Be star. Our
observations show a long-term spin-up trend, with most of the spin-up occurring
during the outbursts. From the measured spin-up rates and inferred luminosities
we conclude that an accretion disk is present during the outbursts.
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No Label
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No Label
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astro-ph/0309087
|
Julian Pittard
|
The Formation of Broad Emission Line Regions in Supernova-QSO Wind
Interactions II. 2D Calculations
|
astro-ph
|
One aspect of supernova remnant evolution that is relatively unstudied is the
influence of an AGN environment. A high density ambient medium and a nearby
powerful continuum source will assist the cooling of shocked ejecta and
swept-up gas. Motion of the surrounding medium relative to the remnant will
also affect the remnant morphology. In an extension to previous work we have
performed 2D hydrodynamical calculations of SNR evolution in an AGN
environment, and have determined the evolutionary behaviour of cold gas in the
remnant. The cold gas will contribute to the observed broad line emission in
AGNs, and we present preliminary theoretical line profiles from our
calculations. A more detailed comparison with observations will be performed in
future work. The SNR-AGN interaction may be useful as a diagnostic of the AGN
wind.
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No Label
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No Label
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astro-ph/9712185
|
John K. Cannizzo
|
Accretion Disks in Transient Systems
|
astro-ph
|
I review recent advances in our understanding of accretion disks in transient
systems - the dwarf novae and the soft X-ray transients. The primary theme will
be the ongoing development of theory in response to the observations. The
accretion disk limit cycle model appears to provide a unifying framework within
which we may begin to understand what is seen in different types of interacting
binary stars, and also to constrain parameters which enter into the theory.
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No Label
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No Label
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astro-ph/0303003
|
Jeremy S. Heyl
|
Accretion limits the compactness of static stars
|
astro-ph
|
General relativity limits the compactness of static stars. If the pressure of
the fluid is positive and the density decreases with distance from the center,
the value of the circumferential radius of the star must be greater than (9/4)
G M/c^2, or equivalently the redshift of the surface must be less than two. If
constraints on the equation of state of the material are relaxed, general
relativity alone does not restrict the redshift of a static stellar surface.
However, because black hole candidates in the universe generally accrete
material from their environs, the process of accretion provides upper limits on
the redshift of a astrophysical black-hole candidates.
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No Label
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No Label
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astro-ph/0504426
|
Mary Barsony
|
A Mid-Infrared Imaging Survey of Embedded Young Stellar Objects in the
Rho Ophiuchi Cloud Core
|
astro-ph
|
Results of a comprehensive, new, ground-based mid-infrared imaging survey of
the young stellar population of the Rho Ophiuchi cloud are presented. Data were
acquired at the Palomar 5-m and at the Keck 10-m telescopes with the MIRLIN and
LWS instruments, at 0.25 arcsec and 0.25 arcsec resolutions, respectively. Of
172 survey objects, 85 were detected. Among the 22 multiple systems observed,
15 were resolved and their individual component fluxes determined. A plot of
the frequency distribution of the detected objects with SED spectral slope
shows that YSOs spend ~400,000 yr in the Flat Spectrum phase, clearing out
their remnant infall envelopes. Mid-infrared variability is found among a
significant fraction of the surveyed objects, and is found to occur for all SED
classes with optically thick disks. Large-amplitude near-infrared variability,
also found for all SED classes with optically thick disks, seems to occur with
somewhat higher frequency at the earlier evolutionary stages. Although a
general trend of mid-infrared excess and NIR veiling exists proceeding through
SED classes, with Class I objects generally exhibiting K-veilings > 1, Flat
Spectrum objects with K-veilings > 0.58, and Class III objects with K-veilings
=0, Class II objects exhibit the widest range of K-band veiling values, 0-4.5.
However, the highly variable value of veiling that a single source can exhibit
in any of the SED classes in which active disk accretion can take place is
striking, and is direct observational evidence for highly time-variable
accretion activity in disks. Finally, by comparing mid-infrared vs.
near-infrared excesses in a subsample with well-determined effective
temperatures and extinction values, disk clearing mechanisms are explored. The
results are consistent with disk clearing proceeding from the inside-out.
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No Label
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No Label
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astro-ph/9904333
|
Inogamov Nail
|
Spread of Matter Over a Neutron-Star Surface During Disk Accretion
|
astro-ph
|
Disk accretion onto a slowly rotating neutron star with a weak magnetic field
$H < 3\times 10^8$ gauss is considered in a wide range of luminosities $1/100 <
L/L_{edd} < 1,$ where $L_{edd}$ is the Eddington luminosity. We construct a
theory for the deceleration of rotation and the spread of matter over the
stellar surface in the shallow-water approximation. The rotation slows down due
to friction against the dense underlying layers. The deceleration of Keplerian
rotation and the energy release take place on the stellar surface in a
latitudinal belt whose meridional width rises with increasing $L.$ The combined
effect of centrifugal force and radiation pressure gives rise to two
latitudinal rings of enhanced brightness which are symmetric around the equator
in the upper and lower hemispheres. They lie near the edges of differentially
rotating and radiating upper and lower belts. The bright rings shift from the
equatorial zone to higher latitudes when the luminosity $L$ rises. The ring
zones are characterized by a minimum surface density and, accordingly, by a
maximum meridional spread velocity. At a low accretion rate and luminosity, the
released energy is removed through the comptonization of low-frequency photons.
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No Label
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No Label
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astro-ph/0110051
|
Elena Pian
|
Optical Observations of Afterglows
|
astro-ph
|
Optical, infrared and ultraviolet observations of GRB fields have allowed
detection of counterparts and host galaxies of the high energy transients, thus
crucially contributing to our present knowledge of the GRB phenomenon.
Measurements of afterglow variable emission, polarized light, redshifted
absorption and emission line spectra, as well as host galaxies brightnesses and
colors have clarified many fundamental issues related to the radiation
mechanisms and environments of GRBs, setting the background towards disclosing
the nature of their progenitors.
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No Label
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No Label
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astro-ph/0104196
|
F. P. Israel
|
CI and CO in the Spiral Galaxies NGC 6946 and M 83
|
astro-ph
|
Multitransition 12CO, 13CO and 492 GHz [CI] measurements of the late-type
spiral galaxies NGC 6946 and M 83 (NGC 5236) show pronounced molecular gas
concentrations in rapid solid-body rotation within a few hundred parsec from
both nuclei. Their 12CO, 13CO and [CI] relative intensities are nearly
identical. However, the very different [CII] intensities imply that the
physical conditions are not. The slow decrease of 12CO intensities with
increasing rotational level marks the presence of significant amounts of warm
and dense molecular gas in both galaxy centers. Detailed modelling indicates
that both galaxiy centers contain at least two distinct molecular components: a
warm and dense component with T(kin) = 30-60 K, n(H2) = 3000-10 000 cc, and a
more tenuous hotter component with T(kin) = 100-150 K, n(H2) < 1000 cc). Atomic
carbon column densities exceed CO column densities by a factor of about 1.5 in
NGC 6946 and about 4 in M 83. Unlike NGC 6946, M 83 contains a significant
amount of molecular hydrogen associated with ionized carbon rather than CO. The
centers of NGC 6946 and M 83 contain nearly identical total (atomic and
molecular) gas masses of about 3 x 10**7 M(sun). Despite their prominence, the
central gas concentrations in these galaxies represent only a few per cent of
the stellar mass in the same volume. The peak face-on gas mass density is much
higher in M 83 (120 M(sun)/pc**2) than in NGC 6946 (45 M(sun)/pc**2). The more
intense starburst in M 83 is associated with a more compact and somewhat hotter
PDR zone than the milder starburst in NGC 6946.
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No Label
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No Label
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astro-ph/9810008
|
Kelvin K. S. Wu
|
The effect of supernova heating on cluster properties and constraints on
galaxy formation models
|
astro-ph
|
Models of galaxy formation should be able to predict the properties of
clusters of galaxies, in particular their gas fractions, metallicities, X-ray
luminosity-temperature relation, temperature function and mass-deposition-rate
function. Fitting these properties places important constaints on galaxy
formation on all scales. By following gas processes in detail, our
semi-analytic model (based on that of Nulsen & Fabian 1997) is the only such
model able to predict all of the above cluster properties. We use realistic gas
fractions and gas density profiles, and as required by observations we break
the self-similarity of cluster structure by including supernova heating of
intracluster gas, the amount of which is indicated by the observed
metallicities. We also highlight the importance of the mass-deposition-rate
function as an independent and very sensitive probe of cluster structure.
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No Label
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No Label
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astro-ph/9609086
|
Marco Tavani
|
Theory of high-energy emission from the pulsar/Be-star system PSR
1259$-$63 I: radiation mechanisms and interaction geometry
|
astro-ph
|
We study the physical processes of the PSR B1259-63 system containing a 47 ms
pulsar orbiting around a Be star in a highly eccentric orbit. Motivated by the
results of a multiwavelength campaign during the January 1994 periastron
passage of PSR B1259-63, we discuss several issues regarding the mechanism of
high-energy emission. Unpulsed power law emission from the this system was
detected near periastron in the energy range 1-200 keV. We find that the
observed high energy emission from the PSR B1259-63 system is not compatible
with accretion or propeller-powered emission. Shock-powered high-energy
emission produced by the pulsar/outflow interaction is consistent with all high
energy observations. By studying the evolution of the pulsar cavity we
constrain the magnitude and geometry of the mass outflow outflow of the Be
star. The pulsar/outflow interaction is most likely mediated by a collisionless
shock at the internal boundary of the pulsar cavity. The system shows all the
characteristics of a {\it binary plerion} being {\it diffuse} and {\it compact}
near apastron and periastron, respectively. The PSR B1259-63 cavity is subject
to different radiative regimes depending on whether synchrotron or inverse
Compton (IC) cooling dominates the radiation of electron/positron pairs
advected away from the inner boundary of the pulsar cavity. The highly
non-thermal nature of the observed X-ray/gamma-ray emission near periastron
establishes the existence of an efficient particle acceleration mechanism
within a timescale shown to be less than $\sim 10^2-10^3$ s. A synchrotron/IC
model of emission of e\pm-pairs accelerated at the inner shock front of the
pulsar cavity and adiabatically expanding in the MHD flow provides an excellent
explanation of the observed time variableX-ray flux and spectrum from the PSR
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No Label
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No Label
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astro-ph/9903466
|
Weidong Li
|
The Type Ia Supernova 1997br in ESO 576-G40
|
astro-ph
|
The peculiar type Ia supernova SN 1997br in ESO 576-G40 was extensively
observed at Beijing Astronomical Observatory and Lick Observatory. In this
paper, we present and discuss the BVRI photometry and the spectra collected
over 3 months, beginning 9 days before maximum brightness. The light curves of
SN 1997br are similar to those of SN 1991T, with slow decline rates after the B
maximum. Well-sampled data before the B maximum show unambiguously that SN
1997br rises more slowly and has a wider peak than normal type Ia supernovae.
The optical color evolution of SN 1997br is also similar to that of SN 1991T.
We estimate the extinction of SN 1997br to be E(B-V) = 0.35+/-0.10 mag by
comparing its BVRI light curves with those of SN 1991T and by measureing the
equivalent width of interstellar Na I D absorption lines. We have conducted a
thorough comparison of the spectroscopic evolution of SN 1997br, SN 1991T, and
SN 1994D. Although SN 1997br is generally very similar to SN 1991T, it shows
some interesting differences at various epoches. Spectra of SN 1997br seem to
indicate an earlier transition to the dominant phase of Fe-peak elements after
the B maximum. Si II lines in SN 1997br show a very short duration after the B
maximum. We discuss the implications of our observations of SN 1997br for
models of type Ia supernovae. Specifically, we suggest that some SNe Ia may
result from decelerated detonations of white dwarfs.
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No Label
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No Label
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astro-ph/0406321
|
Scott M. Ransom
|
Orbital Modulation of the Apparent Dispersion Measure Towards PSR
J0737-3039A
|
astro-ph
|
The paper was withdrawn because the effect is due to the data reduction
methodology. See the text file for details.
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No Label
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No Label
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astro-ph/0302160
|
Asantha R. Cooray
|
Occultation Searches for Kuiper Belt Objects
|
astro-ph
|
The occultation of background stellar sources by foreground Kuiper Belt
Objects (KBOs) can be used to survey physical properties of the KBO population.
We discuss statistics related to a KBO occultation survey, such as the event
duration distribution, and suggest that occultation searches can be effectively
used to probe the KBO size distribution below 10 km. In particular, we suggest
that occultation surveys may be best suited to search for a turnover radius in
the KBO size distribution due to collisions between small-size objects. For
occultation surveys that monitor stellar sources near the ecliptic over a few
square degrees, with time sampling intervals of order 0.1 sec and sensitivity
to flux variations of a few percent or more, a turnover radius between 0.1 and
1.0 km can be probed. While occultation surveys will probe the low-radius limit
and imaging surveys will detect KBOs of size 100 km or more, statistics of
objects with sizes in the intermediate range of around 1 km to 100 km will
likely remain unattainable.
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No Label
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No Label
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0707.4127
|
Simon Verley
|
Star formation in isolated AMIGA galaxies: dynamical influence of bars
|
astro-ph
|
Star formation depends strongly both on the local environment of galaxies,
and on the internal dynamics of the interstellar medium. To disentangle the two
effects, we obtained, in the framework of the AMIGA project, Ha and Gunn r
photometric data for more than 200 spiral galaxies lying in very low-density
regions of the local Universe. We characterise the Ha emission, tracing current
star formation, of the 45 largest and less inclined galaxies observed for which
we estimate the torques between the gas and the bulk of the optical matter. We
could subsequently study the Ha morphological aspect of these isolated spiral
galaxies. Using Fourier analysis, we focus on the modes of the spiral arms and
also on the strength of the bars, computing the torques between the gas and
newly formed stars (Ha) and the bulk of the optical matter (Gunn r). We
interpret the various bar/spiral morphologies observed in terms of the secular
evolution experienced by galaxies in isolation. We also classify the different
spatial distributions of star forming regions in barred galaxies. The observed
frequency of particular patterns brings constraints on the lifetime of the
various evolution phases. We propose an evolutive sequence accounting for the
transitions between the different phases we could observe. Isolated galaxies
appear not to be preferentially barred or unbarred. Through numerical
simulations, trying to fit the Ha distributions yields constraints on the star
formation law, which is likely to differ from a genuine Schmidt law. In
particular, it is probable that the relative velocity of the gas in the bar
also needs to be taken into account.
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No Label
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No Label
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astro-ph/0211269
|
Steven W. Barwick
|
Physics and Operation of the AMANDA-II High Energy Neutrino Telescope
|
astro-ph
|
This paper briefly describes the principle of operation and science goals of
the AMANDA high energy neutrino telescope located at the South Pole,
Antarctica. Results from an earlier phase of the telescope, called AMANDA-B10,
demonstrate both reliable operation and the broad astrophysical reach of this
device, which includes searches for a variety of sources of ultrahigh energy
neutrinos: generic point sources, Gamma-Ray Bursts and diffuse sources. The
predicted sensitivity and angular resolution of the telescope were confirmed by
studies of atmospheric muon and neutrino backgrounds. We also report on the
status of the analysis from AMANDA-II, a larger version with far greater
capabilities. At this stage of analysis, details of the ice properties and
other systematic uncertainties of the AMANDA-II telescope are under study, but
we have made progress toward critical science objectives. In particular, we
present the first preliminary flux limits from AMANDA-II on the search for
continuous emission from astrophysical point sources, and report on the search
for correlated neutrino emission from Gamma Ray Bursts detected by BATSE before
decommissioning in May 2000. During the next two years, we expect to exploit
the full potential of AMANDA-II with the installation of a new data acquisition
system that records full waveforms from the in-ice optical sensors.
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No Label
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No Label
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astro-ph/9802307
|
Krzysztof Stanek
|
Using the DIRBE/IRAS All-Sky Reddening Map To Select Low-Reddening
Windows Near the Galactic Plane
|
astro-ph
|
Recently Schlegel, Finkbeiner & Davis published an all-sky reddening map
based on the COBE/DIRBE and IRAS/ISSA infrared sky surveys. Using the reddening
map of Baade's Window and sample of 19 low-latitude ($|b|<5\deg$) Galactic
globular clusters I find that the DIRBE/IRAS reddening map overestimates
$E(B-V)$ at low galactic latitudes by a factor of $\sim 1.35$. I also
demonstrate the usefulness of this high resolution map for selecting
low-reddening windows near the Galactic plane.
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No Label
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No Label
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astro-ph/0703598
|
Hua Feng
|
Spectral Evolution of NGC 1313 X-2: Evidence Against The Cool Disk Model
|
astro-ph
|
The presence of a cool multicolor disk component with an inner disk
temperature kT=0.1~0.3 keV at a luminosity L>10^40 erg/s has been interpreted
as evidence that the ultraluminous X-ray source NGC 1313 X-2 harbors an
intermediate-mass black hole (IMBH). The temperature of a disk component should
vary with luminosity as $L\propto T^4$. However, upon investigating the
spectral evolution with multiple XMM-Newton observations, we found that the
cool disk component failed to follow this relation with a confidence level of
0.999964. Indeed, the luminosity decreases as the temperature increases, and
the luminosities at high temperatures are more than an order of magnitude less
than expected from the $L\propto T^4$ extrapolation of luminosities at low
temperatures. This places a strong constraint against the validity of modeling
the X-ray spectra of NGC 1313 X-2 as emission from the accretion disk of an
IMBH. The decrease in luminosity with increasing temperature of the soft
component follows the trend suggested by a model in which the soft emission
arises from an outflow from a stellar-mass black hole with super-Eddington
accretion viewed along the symmetry axis. Alternatively, the spectra can be
adequately fitted by a p-free disk model with kT=~2 keV and p=~0.5. The
spectral evolution is consistent with the $L\propto T^4$ relation and appears
to be a high luminosity extension of the L-kT relation of Galactic black holes.
This, again, would suggest that the emission is from a super-Eddington
accreting stellar mass black hole.
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No Label
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No Label
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astro-ph/0502148
|
Tsvi Piran
|
The distances of short-hard GRBs and the SGR connection
|
astro-ph
|
We present a search for nearby (D<100 Mpc) galaxies in the error boxes of six
well-localized short-hard gamma-ray bursts (GRBs). None of the six error boxes
reveals the presence of a plausible nearby host galaxy. This allows us to set
lower limits on the distances and, hence, the isotropic-equivalent energy of
these GRBs. Our lower limits are around $1 \times 10^{49}$ erg (at $2\sigma$
confidence level); as a consequence, some of the short-hard GRBs we examine
would have been detected by BATSE out to distances greater than 1 Gpc and
therefore constitute a bona fide cosmological population. Our search is
partially motivated by the December 27, 2004 hypergiant flare from SGR 1806-20,
and the intriguing possibility that short-hard GRBs are extragalactic events of
a similar nature. Such events would be detectable with BATSE to a distance of
\~50 Mpc, and their detection rate should be comparable to the actual BATSE
detection rate of short-hard GRBs. The failure of our search, by contrast,
suggests that such flares constitute less than 15% of the short-hard GRBs (<40%
at 95% confidence). We discuss possible resolutions of this discrepancy.
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No Label
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astro-ph/9609113
|
Edward Ajhar
|
The SBF Survey of Galaxy Distances. I. Sample Selection, Photometric
Calibration, and the Hubble Constant
|
astro-ph
|
We describe a program of surface brightness fluctuation (SBF) measurements
for determining galaxy distances. This paper presents the photometric
calibration of our sample and of SBF in general. Basing our zero point on
observations of Cepheid variable stars, we find that the absolute SBF magnitude
in the Kron-Cousins I band correlates well with the mean (V-I)o color of a
galaxy according to
M_Ibar = (-1.74 +/- 0.07) + (4.5 +/- 0.25) [ (V-I)o - 1.15 ]
for 1.0 < (V-I) < 1.3. This agrees well with theoretical estimates from
stellar population models. Comparisons between SBF distances and a variety of
other estimators, including Cepheid variable stars, the Planetary Nebula
Luminosity Function (PNLF), Tully-Fisher (TF), Dn-sigma, SNII, and SNIa,
demonstrate that the calibration of SBF is universally valid and that SBF error
estimates are accurate. The zero point given by Cepheids, PNLF, TF (both
calibrated using Cepheids), and SNII is in units of Mpc; the zero point given
by TF (referenced to a distant frame), Dn-sigma and SNIa is in terms of a
Hubble expansion velocity expressed in km/s. Tying together these two zero
points yields a Hubble constant of H_0 = 81 +/- 6 km/s/Mpc. As part of this
analysis, we present SBF distances to 12 nearby groups of galaxies where
Cepheids, SNII, and SNIa have been observed.
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No Label
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astro-ph/9610199
|
Gillian Wilson
|
The Faint End of the Galaxy Luminosity Function In Moderate Redshift
Clusters
|
astro-ph
|
We present deep two-colour photometry of two rich clusters at z=0.18, A665
and A1689. We use these data to construct number counts as a function of
magnitude in the two fields. By combining these counts with similar
observations from a large area field survey we subtract the field contamination
statistically to produce luminosity functions for the two clusters. Great care
has been taken to achieve agreement between the photometry of these two
samples. The cluster data are complete to a limiting magnitude of I=22.5 or an
absolute magnitude in the cluster of I= -18.0 ($M^\star+5$). The luminosity
functions of both clusters are well described by a Gaussian function for the
bright galaxies, combined with a Schechter function at the faint end, similar
to that required to fit the luminosity function in local clusters. The slope at
the faint end of the Schechter function in both clusters is extremely steep in
V, alpha ~ -2. A shallower slope is seen to the limit of the I data, indicating
that the cluster population is rapidly blueing as we reach fainter. The
excellent agreement between the form of the luminosity function in our two
distant clusters, as well as agreement with the luminosity function given by
Driver et al (1994) for a single z=0.21 cluster, indicates that this faint blue
population is a general constituent of distant clusters. We compare our results
with those from studies of local clusters. Depending upon the degree of fading
(or disruption) of these faint blue galaxies,we tentatively identify their
remnants with the low surface brightness dwarf galaxies which are the dominant
population in local clusters. We discuss the possible role of this population
as the source of most of the X-ray gas in rich clusters.
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astro-ph/9809042
|
Chris Churchill
|
The Multiple Phases of Interstellar and Halo Gas in a Possible Group of
Galaxies at z~1
|
astro-ph
|
We used HIRES/Keck profiles (R=6 km/s) of MgII and FeII in combination with
FOS/HST spectra (R=230 km/s) to place constraints on the physical conditions
(metallicities, ionization conditions, and multi-phase distribution) of
absorbing gas in three galaxies at z=0.9254, 0.9276, and 0.9343 along the line
of sight to PG 1206+459. The chemical and ionization species covered in the
FOS/HST spectra are HI, SiII, CII, NII, FeIII, SiIII, SiIV, NIII, CIII, CIV,
SVI, NV, and OVI, with ionization potentials ranging from 13.6 to 138 eV. The
multiple MgII clouds exhibit complex kinematics and the CIV, NV and OVI are
exceptionally strong in absorption. We assumed that the MgII clouds are
photoionized by the extra-galactic background and determined the allowed ranges
of their physical properties as constrained by the absorption strengths in the
FOS spectra. A main result of this paper is that the low resolution spectra can
provide meaningful constraints on the physical conditions of the MgII clouds,
including allowed ranges of cloud to cloud variations within a system. We find
that the MgII clouds, which have a typical size of ~100 pc, give rise to the
SiIV, the majority of which arises in a single, very large (~5 kpc), higher
ionization cloud. However, the MgII clouds cannot account for the strong CIV,
NV, and OVI absorption. We conclude that the MgII clouds are embedded in
extended (10-20 kpc), highly ionized gas that gives rise to CIV, NV, OVI; these
are multi-phase absorption systems. The high ionization phases have near-solar
metallicity and are consistent with Galactic-like coronae surrounding the
individual galaxies, as opposed to a very extended common "halo" encompassing
all three galaxies.
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No Label
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astro-ph/0605139
|
Hector G. Arce
|
The Evolution of Outflow-Envelope Interactions in Low-Mass Protostars
|
astro-ph
|
We present multi-line and continuum observations of the circumstellar
environment within 10^4 AU of a sample of protostars to investigate how the
effects of outflows on their immediate environment changes over time. 12CO(1-0)
emission probes the high-velocity molecular outflows near the protostars and
demonstrate that the outflow opening angle widens as the nascent star evolves.
Maps of the 13CO(1-0) and HCO+(1-0) outflow emission show that protostellar
winds erode the circumstellar envelope through the entrainment of the outer
envelope gas. The spatial and velocity distribution of the dense circumstellar
envelope, as well as its mass, is traced by the C18O(1-0) emission and also
displays evolutionary changes. We show that outflows are largely responsible
for these changes, and propose an empirical model for the evolution of
outflow-envelope interactions. In addition, some of the outflows in our sample
appear to affect the chemical composition of the surrounding environment,
enhancing the HCO+ abundance. Overall, our results confirm that outflows play a
major role in the star formation process through their strong physical and
chemical impacts on the environments of the young protostars.
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No Label
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No Label
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astro-ph/9801257
|
Paul N. Arendt Jr.
|
The Shape of Pulsar Polar Caps
|
astro-ph
|
Rotation distorts the vacuum magnetic field of a pulsar from that of a simple
dipole. The effect is particularly strong close to the light cylinder, but also
affects the field close to the stellar surface. We find the shape and locus of
the field lines which just close at the light cylinder. Their footpoints define
the pulsar polar cap. We find this cap is asymmetric and distorted. We also
find that the polar cap is not strongly elliptical. This result disagrees with
calculations based on the non-rotating dipole field. We present our numerical
results, and discuss consequences for interpretation of mean profiles and for
pulsar statistics.
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No Label
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astro-ph/0002422
|
Rupert A. C. Croft
|
Simulating the effects of intergalactic grey dust
|
astro-ph
|
Using a high-resolution cosmological hydrodynamic simulation, we present a
method to constrain extinction due to intergalactic grey dust based on the
observed magnitudes of distant Type IA supernovae. We apply several simple
prescriptions to relate the intergalactic dust density to the gas density in
the simulation, thereby obtaining dust extinctions that may be directly
compared to the observed distribution of supernova magnitudes. Our analysis is
sensitive to the spatial distribution of grey dust, but is not dependent on its
intrinsic properties such as its opacity or grain size. We present an
application of our technique to the supernova data of Perlmutter et al., who
find that their high redshift sample is ~0.2 magnitudes fainter than the
expectation for a non-accelerating, low-density universe. We find that for grey
dust to be responsible, it must be distributed quite smoothly, e.g., tracing
intergalactic gas. More realistic dust distributions, such as dust tracing the
metal density, are inconsistent with observations at the 1.5-2 sigma level.
Upcoming observations and improved modelling of the dust distribution should
lead to stronger constraints on intergalactic grey dust extinction.
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No Label
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No Label
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0709.2861
|
Michele Cappellari
|
Supermassive black holes from OASIS and SAURON integral-field kinematics
|
astro-ph
|
Supermassive black holes are a key element in our understanding of how
galaxies form. Most of the progress in this very active field of research is
based on just ~30 determinations of black hole mass, accumulated over the past
decade. We illustrate how integral-field spectroscopy, and in particular our
OASIS modeling effort, can help improve the current situation.
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No Label
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No Label
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astro-ph/9811250
|
Luis F. Rodriguez
|
The surroundings of the superluminal source GRS 1915+105
|
astro-ph
|
We have carried out radio studies of the surroundings of the superluminal
microquasar GRS~1915+105. Our main goal was to understand the possible relation
of GRS~1915+105 with two infrared/radio sources that appear symmetrically
located with respect to GRS~1915+105 and aligned with the position angle of the
relativistic ejecta. We have also studied a nearby supernova remnant to test if
the event that created the remnant could have been the progenitor of this hard
X-ray binary.
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No Label
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No Label
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astro-ph/0203410
|
Patrick Cote
|
Palomar 13: An Unusual Stellar System in the Galactic Halo
|
astro-ph
|
We have measured Keck/HIRES radial velocities for 30 candidate red giants in
the direction of Palomar 13: an object traditionally cataloged as a compact,
low-luminosity globular cluster. From a sample of 21 confirmed members, we find
a systemic velocity of 24.1 km/s and a projected, intrinsic velocity dispersion
of 2.2 km/s. Although small, this dispersion is several times larger than that
expected for a globular cluster of this luminosity and central concentration.
Taken at face value, this dispersion implies a mass-to-light ratio of ~ 40 (in
solar units) based on the best-fit King-Michie model. The surface density
profile of Palomar 13 also appears to be anomalous among Galactic globular
clusters -- depending upon the details of background subtraction and
model-fitting, Palomar 13 either contains a substantial population of
"extra-tidal" stars, or it is far more spatially extended than previously
suspected. The full surface density profile is equally well-fit by a
King-Michie model having a high concentration and large tidal radius, or by a
NFW model. We examine -- and tentatively reject -- a number of possible
explanations for the observed characteristics of Palomar 13 (e.g., velocity
"jitter" among the red giants, spectroscopic binary stars, non-standard mass
functions, modified Newtonian dynamics), and conclude that the two most
plausible scenarios are either catastrophic heating during a recent
perigalacticon passage, or the presence of a massive dark halo. Thus, the
available evidence suggests that Palomar 13 is either a globular cluster which
is now in the process of dissolving into the Galactic halo, or a faint,
dark-matter-dominated stellar system (ABRIDGED).
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No Label
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No Label
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astro-ph/9608120
|
Eric Deutsch
|
Photometry of GX 349+2: Evidence for a 22-hour Period
|
astro-ph
|
The intense galactic X-ray source GX 349+2 (Sco X-2) belongs to the class of
persistently bright low-mass X-ray binaries called Z-sources. GX 349+2,
although observed in X-rays for more than 30 years, has only recently been
optically identified with a 19th mag star. Of the six known Z-sources, only two
(Sco X-1 and Cyg X-2) have been studied in the optical. It has been suggested
that Z-sources as a group are characterized by evolved companions and
correspondingly long orbital periods (Sco X-1, P=0.8 d; Cyg X-2, P=9.8 d).
Recently Southwell et al. (1996) have presented spectroscopic observations of
GX 349+2 suggesting a 14 d orbital period. We have obtained broadband
photometry of the system on six consecutive nights, and find a statistically
significant 21.85 +/- 0.4 h (3 sigma) period of 0.14 mag half-amplitude,
superposed on erratic flickering typical of Sco X-1 type objects. As with other
Z-sources, caution will be needed to insure that the variations are truly
periodic, and not simply due to chaotic variability observed over a relatively
short time span. Depending on the origin of the brightness variations, our
proposed period could be either the orbital or half the orbital period. If our
period is confirmed, then the nature of the 14 d spectroscopic variation found
by Southwell et al. (1996) is unclear. There is evidence that the mass function
of GX 349+2 is similar to that of Sco X-1.
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No Label
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No Label
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0806.2530
|
Martin Durant
|
SWIFT J1753.5-0127: a surprising optical/X-ray cross-correlation
function
|
astro-ph
|
We have conducted optical and X-ray simultaneous observations of SWIFT
J1753.5-0127 with RXTE and ULTRACAM, while the system persisted in its
relatively bright low/hard state. In the cross-correlation function (CCF), we
find that the optical leads the X-rays by a few seconds with a broad negative
peak, and has a smaller positive peak at positive lags. This is markedly
different from what was seen for the similarly interesting system XTE
J1118+480, and the first time such a correlation function has been so clearly
measured. Furthermore, there appears to be a significant variation of the
correlation with X-ray energy. We suggest a physical scenario for its origin.
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No Label
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astro-ph/9409071
|
Harry Ferguson
|
The Contribution of Low Surface-Brightness Galaxies to Faint Galaxy
Counts
|
astro-ph
|
Low Surface-Brightness (LSB) galaxies are severely underrepresented in
surveys used to define the local luminosity function (LF), but could it in
principle be detected in deep surveys. To explore the possible contribution of
such objects to faint galaxy counts, we construct catalogs of simulated
non-evolving galaxies drawn from a multivariate distribution of galaxy
luminosities, central surface brightnesses, bulge/disk ratios and
spectral-energy distributions. We compare two models dominated by LSB galaxies
to a standard non-evolving model. Model galaxies are convolved with seeing and
selected in a manner that closely matches real surveys. For each model we
compute the local LF, HI mass function, number counts, redshift and color
distributions. We find it possible to include a large population of LSB
galaxies and incorporate a steep faint-end slope of the LF in our simulations
without violating the constraints on the local LF or HI mass function. For $q_0
= 0.5$, the most favorable model matches the counts to B=23, but falls short of
the observations at fainter magnitudes. The colors and redshift distributions
remain roughly consistent with observations to B=24. The most serious
discrepancy with observations is in the distribution of $r_e$ at faint
magnitudes, suggesting that the model contains too many LSB galaxies.
Nevertheless, the results suggest that LSB galaxies could be a significant
contributor to faint-galaxy counts, reducing the need for such extreme models
of galaxy evolution as rapid merging or bursting dwarf galaxies.
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No Label
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astro-ph/0006155
|
Douglas L. Tucker
|
Compact Groups of Galaxies in the Las Campanas Redshift Survey
|
astro-ph
|
We have recently extracted a catalog of compact groups of galaxies (CGs) from
the Las Campanas Redshift Survey. This catalog of Las Campanas Compact Groups
(LCCGs) contains 76 CGs with a median redshift of z_med = 0.08. The physical
properties of these CGs are similar to those from the Hickson (1982) and the
Barton et al. (1996) catalogs. Here, we present an atlas of our catalog and
briefly describe its general properties.
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No Label
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No Label
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astro-ph/0610711
|
Carlotta Gruppioni Dr
|
Galaxy and AGN Evolution in the MIR: a combined Spitzer and X-ray view
|
astro-ph
|
A proper analysis of the evolution of sources emitting in the Mid-Infrared is
strongly dependent on their broad-band spectral properties (SEDs) at different
redshifts and luminosities and on a reliable classification allowing to
disentangle AGN from star-formation activity. The diagnostic diagrams based on
the optical line ratios are often ambiguous and/or misleading not allowing a
proper separation of the galaxy/AGN populations. Thanks to the combination of
deep Spitzer and X-rays data a much better census of the hidden AGN activity
and dust-obscured star-forming galaxies can be obtained, constraining galaxy
and AGN evolutionary models.
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No Label
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No Label
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0711.3414
|
Adam D. Myers
|
Developing and Deploying Advanced Algorithms to Novel Supercomputing
Hardware
|
astro-ph
|
The objective of our research is to demonstrate the practical usage and
orders of magnitude speedup of real-world applications by using alternative
technologies to support high performance computing. Currently, the main barrier
to the widespread adoption of this technology is the lack of development tools
and case studies that typically impede non-specialists that might otherwise
develop applications that could leverage these technologies. By partnering with
the Innovative Systems Laboratory at the National Center for Supercomputing, we
have obtained access to several novel technologies, including several
Field-Programmable Gate Array (FPGA) systems, NVidia Graphics Processing Units
(GPUs), and the STI Cell BE platform. Our goal is to not only demonstrate the
capabilities of these systems, but to also serve as guides for others to follow
in our path. To date, we have explored the efficacy of the SRC-6 MAP-C and
MAP-E and SGI RASC Athena and RC100 reconfigurable computing platforms in
supporting a two-point correlation function which is used in a number of
different scientific domains. In a brute force test, the FPGA based
single-processor system has achieved an almost two orders of magnitude speedup
over a single-processor CPU system. We are now developing implementations of
this algorithm on other platforms, including one using a GPU. Given the
considerable efforts of the cosmology community in optimizing these classes of
algorithms, we are currently working to implement an optimized version of the
basic family of correlation functions by using tree-based data structures.
Finally, we are also exploring other algorithms, such as instance-based
classifiers, power spectrum estimators, and higher-order correlation functions
that are also commonly used in a wide range of scientific disciplines.
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No Label
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No Label
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astro-ph/0511599
|
Agnieszka Slowikowska
|
Polarization characteristics of the Crab pulsar's giant radio pulses at
HFCs phases
|
astro-ph
|
We discuss our recent discovery of the giant radio emission from the Crab
pulsar at its high frequency components (HFCs) phases and show the polarization
characteristic of these pulses. This leads us to a suggestion that there is no
difference in the emission mechanism of the main pulse (MP), interpulse (IP)
and HFCs. We briefly review the size distributions of the Crab giant radio
pulses (GRPs) and discuss general characteristics of the GRP phenomenon in the
Crab and other pulsars.
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No Label
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No Label
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0710.4934
|
Giuseppe Lavagetto
|
Spectral analysis of LMC X-2 with XMM/Newton: unveiling the emission
process in the extragalactic Z-source
|
astro-ph
|
We present the results of the analysis of an archival observation of LMC X-2
performed with XMM/Newton. The spectra taken by high-precision instruments have
never been analyzed before. We find an X-ray position for the source that is
inconsistent with the one obtained by ROSAT, but in agreement with the Einstein
position and that of the optical counterpart. The correlated spectral and
timing behaviour of the source suggests that the source is probably in the
normal branch of its X-ray color-color diagram. The spectrum of the source can
be fitted with a blackbody with a temperature 1.5 keV plus a disk blackbody at
0.8 keV. Photoelectric absorption from neutral matter has an equivalent
hydrogen column of 4 x 10^{20} cm^{-2}. An emission line, which we identify as
the O VIII Lyman alpha line, is detected, while no feature due to iron is
detected in the spectrum. We argue that the emission of this source can be
straightforwardly interpreted as a sum of the emission from a boundary layer
between the NS and the disc and a blackbody component coming from the disc
itself. Other canonical models that are used to fit Z-sources do not give a
satisfactory fit to the data. The detection of the O VIII emission line (and
the lack of detection of lines in the iron region) can be due to the fact that
the source lies in the Large Magellanic Cloud.
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No Label
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astro-ph/9410069
|
Paul Sommers
|
In Search of a Source for the 320 Eev Fly's Eye Cosmic Ray
|
astro-ph
|
The 320 EeV air shower detected by the Fly's Eye poses an important problem.
Careful analysis of pathlength limitations for the possible particle types due
to cosmic background radiation verifies that the particle very likely traveled
less than 50 Mpc from its source. The best candidates for accelerating
particles to such high energies are the very powerful radiogalaxies, however
they are all more than 100 Mpc distant. Our search finds no likely source
within 50 Mpc in the direction from which the particle arrived. This prompts
consideration of less likely astrophysical sources, like M82, as well as
non-standard mechanisms like cosmic string annihilation. It is also conceivable
that the air shower was produced by some non-standard particle whose pathlength
is unlimited because it does not interact with the cosmic background radiation.
A less radical alternative is that relatively strong magnetic fields deflected
the particle's path through a large angle, so it could have originated at a
nearby radiogalaxy at an earlier time of strong activity.
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astro-ph/9902042
|
Roland E. Allen
|
Four Testable Predictions of Instanton Cosmology
|
astro-ph
|
A new cosmological model makes the following predictions: (1) The
deceleration parameter $q_{0}$ is approximately zero. (2) The mass density
parameter $\Omega_{m}$ is less than 1. (3) The universe is spatially closed,
but is asymptotically flat as $t\to\infty$, regardless of its matter content.
(4) The age of the universe is approximately 15 Gyr if the Hubble parameter $h$
is approximately 0.65.
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No Label
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0712.0911
|
Christian Holler
|
Implementation of an Optimised Cassegrain System for Radio Telescopes
|
astro-ph
|
We present the antenna design for a radio interferometer, the Arcminute
Microkelvin Imager, together with its beam pattern measurement. Our aim was to
develop a low-cost system with high aperture efficiency and low ground-spill
across the frequency range 12-18GHz. We use a modified cassegrain system
consisting of a commercially-available paraboloidal primary mirror with a
diameter of 3.7m, and a shaped secondary mirror. The secondary mirror is
oversized with respect to a ray-optics design and has a surface that is bent
towards the primary near its outer edge using a square term for the shaping.
The antennas are simple to manufacture and therefore their cost is low. The
design increased the antenna gain by approximately 10 per cent compared to a
normal Cassegrain system while still maintaining low contamination from
ground-spill and using a simple design for the horn.
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astro-ph/0001310
|
Ken'ichi Kikuchi
|
Detection of an X-Ray Hot Region in the Virgo Cluster of Galaxies with
ASCA
|
astro-ph
|
Based on mapping observations with ASCA, an unusual hot region with a spatial
extent of 1 square degree was discovered between M87 and M49 at a center
coordinate of R. A. = 12h 27m 36s and Dec. = $9^\circ18'$ (J2000). The X-ray
emission from the region has a 2-10 keV flux of $1 \times 10^{-11}$ ergs
s$^{-1}$ cm$^{-2}$ and a temperature of $kT \gtrsim 4$ keV, which is
significantly higher than that in the surrounding medium of $\sim 2$ keV. The
internal thermal energy in the hot region is estimated to be $V n k T \sim
10^{60}$ ergs with a gas density of $\sim 10^{-4}$ cm$^{-3}$. A power-law
spectrum with a photon index $1.7-2.3$ is also allowed by the data. The hot
region suggests there is an energy input due to a shock which is probably
caused by the motion of the gas associated with M49, infalling toward the M87
cluster with a velocity $\gtrsim 1000$ km s$^{-1}$.
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astro-ph/0404460
|
Vadim Arefiev
|
Broadband X-ray spectrum of XTE J1550-564 during 2003 outburst
|
astro-ph
|
Results of broadband INTEGRAL and RXTE observations of the Galactic
microquasar XTE J1550-564 during outburst in spring 2003 are presented. During
the outburst the source was found in a canonical low/hard spectral state.
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astro-ph/0109324
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Pierluigi Monaco
|
PINOCCHIO and the hierarchical build-up of dark matter haloes
|
astro-ph
|
We study the ability of PINOCCHIO (PINpointing Orbit-Crossing Collapsed
HIerarchical Objects) to predict the merging histories of dark matter (DM)
haloes, comparing the PINOCCHIO predictions with the results of two large
N-body simulations run from the same set of initial conditions. We focus our
attention on quantities most relevant to galaxy formation and large-scale
structure studies. PINOCCHIO is able to predict the statistics of merger trees
with a typical accuracy of 20 per cent. Its validity extends to higher-order
moments of the distribution of progenitors. The agreement is valid also at the
object-by-object level, with 70-90 per cent of the progenitors cleanly
recognised when the parent halo is cleanly recognised itself. Predictions are
presented also for quantities that are usually not reproduced by semi-analytic
codes, such as the two-point correlation function of the progenitors of massive
haloes and the distribution of initial orbital parameters of merging haloes.
For the accuracy of the prediction and for the facility with which merger
histories are produced, PINOCCHIO provides a means to generate catalogues of DM
haloes which is extremely competitive to large-scale N-body simulations, making
it a suitable tool for galaxy formation and large-scale structure studies.
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astro-ph/0108455
|
Pratik Mazumdar
|
Pachmarhi Array of \v{C}erenkov Telescopes and its Sensitivity
|
astro-ph
|
Pachmarhi Array of \v{C}erenkov Telescopes (PACT) has been designed to search
for celestial TeV $\gamma-$rays using the wavefront sampling technique. PACT,
located at Pachmarhi, (latitude 22$^\circ$ 28$^\prime$ N, longitude 76$^\circ$
26$^\prime$ E, altitude 1075 m) consists of 25 telescopes deployed over an area
of 80 m $\times$ 100m. Each telescopes consists of 7 parabolic reflectors, each
viewed by a fast phototube behind a 3$^\circ$ mask at the focus. The density
and the arrival time of the photons at the PMT are recorded for each shower.
The energy threshold and collection area of the array are estimated, from Monte
Carlo simulations, to be $\sim$ 900 GeV and 10$^5$ m$^2$ respectively. The
accuracy in determination of arrival angle of a shower is estimated to be about
0.1$^\circ$ in the near vertical direction. About 99% of the off-axis hadronic
events could be rejected from directional information alone. Further, at least
75% of the on-axis hadronic events could be rejected using species sensitive
parameters derived from timing and density measurements. These cuts on data to
reject background would retain $\sim$ 44% of the $\gamma-$ray signal. The
sensitivity of the array for a 5$\sigma$ detection of $\gamma-$ray signal at a
threshold energy of 1 TeV has been estimated to be $\sim$ 4.1 $\times$
10$^{-12}$ photons cm$^{-2}$ s$^{-1}$ for an on source exposure of 50 hours.
The PACT set-up has been fully commissioned and is collecting data. The details
of the system parameters and sensitivity will be presented.
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astro-ph/0008080
|
Marco Castellani
|
Antlia: an outskirt Local Group Galaxy
|
astro-ph
|
Deep (I,V-I) and (I,B-I) color-magnitude diagrams (CMDs) of the Antlia dwarf
galaxy, based on Science Verification (SV) data collected with the FORS I
camera on the ESO Very Large Telescope (VLT) are presented.
By adopting the new calibration of the Tip of the Red Giant Branch (TRGB)
provided by Salaris & Cassisi (1998) we estimated that the Antlia distance
modulus is (m-M)o=25.98 with an error of 0.10 mag.
We suggest an improvement of the classical TRGB method based on infrared H
and K magnitudes of TRGB stars. Such a method should overcome the well-known
limit of the "classical" TRGB method when dealing with metal-rich stellar
populations.
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astro-ph/9811074
|
Loredana Bassani
|
A 3 dimensional diagnostic diagram for Seyfert 2s: probing X-ray
absorption and Compton thickness
|
astro-ph
|
We present and discuss a "3-dimensional" diagnostic diagram for Seyfert2
galaxies obtained by means of X-ray and [OIII] data on a large sample of
objects (reported in the Appendix). The diagram shows the Kalpha iron line
equivalent width as a function of both the column density derived from the
photoelectric cutoff and the 2-10 keV flux normalized to the [OIII] optical
line flux (the latter corrected for extinction and assumed to be a true
indicator of the source intrinsic luminosity). We find that the hard X-ray
properties of type 2 objects depend on a single parameter, the absorbing column
density along the line of sight,in accordance with the unified model. The
diagram can be used to identify Compton thick sources and to isolate and study
peculiar objects. From this analysis we have obtained a column density
distribution of Seyfert 2 galaxies which is thought to be a good approximation
of the real distribution. A large population of heavily absorbed objects is
discovered, including many Compton thick candidates. Our results indicate that
the mean Log Nh/cm^(-2)in type 2 Seyferts is 23.5 and that as much as 23-30% of
sources have Nh > 10^24 cm^(-2).
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astro-ph/0001470
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Piotr Popowski
|
Harmonizing the RR Lyrae and Clump Distance Scales - Stretching the
Short Distance Scale to Intermediate Ranges?
|
astro-ph
|
I explore the consequences of making the RR Lyrae and clump giant distance
scales consistent in the solar neighborhood, Galactic bulge and Large
Magellanic Cloud (LMC). I employ two major assumptions: 1) that the absolute
magnitude - metallicity, M_V(RR) - [Fe/H], relation for RR Lyrae stars is
universal, and 2) that absolute I-magnitudes of clump giants, M_I(RC), in
Baade's Window are known (e.g., can be inferred from the local Hipparcos-based
calibration or theoretical modeling). A comparison between the solar
neighborhood and Baade's Window sets M_V(RR) at [Fe/H] = -1.6 in the range
(0.59 +/- 0.05, 0.70 +/- 0.05), somewhat brighter than the statistical parallax
solution. More luminous RR Lyrae stars imply younger ages of globular cluster,
which would be in better agreement with the conclusions from the currently
favored stellar evolution and cosmological models. A comparison between Baade's
Window and the LMC sets the M_I^{LMC}(RC) in the range (-0.33 +/- 0.09, -0.53
+/- 0.09). The distance modulus to the LMC, mu^{LMC}, is between 18.24 +/- 0.08
and 18.44 +/- 0.07. Unlike M_I^{LMC}(RC), this range in mu^{LMC} does NOT
depend on the adopted value of the dereddened LMC clump magnitude,
I_0^{LMC}(RC). I argue that the currently available information is insufficient
to select the correct distance scale with high confidence.
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astro-ph/0112250
|
Koji Mori
|
Chandra reveals the Dynamic Structure of the Inner Crab Nebula
|
astro-ph
|
We present a series of monitoring observations of the Crab Nebula with the
Chandra X-ray Observatory, focusing on the temporal evolution of the structure.
This series of 8 observations, spanning a period of approximately six months,
shows the dynamic nature of the inner X-ray structures. We detected outward
moving ``wisps'' from the recently discovered inner ring seen in optical
observations. We also find that the inner ring itself shows temporal variations
in structure. The torus also appears to be expanding. Such temporal variations
generally match the canonical scenario that an expanding synchrotron nebula
injected from the pulsar is confined by the supernova ejecta.
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astro-ph/0401052
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Annie Robin
|
A synthetic view on structure and evolution of the Milky Way
|
astro-ph
|
Since the Hipparcos mission and recent large scale surveys in the optical and
the near-infrared, new constraints have been obtained on the structure and
evolution history of the Milky Way. The population synthesis approach is a
useful tool to interpret such data sets and to test scenarios of evolution of
the Galaxy. We present here new constraints on evolution parameters obtained
from the Besancon model of population synthesis and analysis of optical and
near-infrared star counts. The Galactic potential is computed
self-consistently, in agreement with Hipparcos results and the observed
rotation curve. Constraints are posed on the outer bulge structure, the warped
and flared disc, the thick disc and the spheroid populations. The model is
tuned to produce reliable predictions in the visible and the near-infrared in
wide photometric bands from U to K. Finally, we describe applications such as
photometric and astrometric simulations and a new classification tool based on
a Bayesian probability estimator, which could be used in the framework of
Virtual Observatories. As examples, samples of simulated star counts at
different wavelengths and directions are also given.
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astro-ph/0311631
|
James E. Taylor
|
The Satellite-substructure Connection
|
astro-ph
|
We describe our recent attempts to model substructure in dark matter halos
down to very small masses, using a semi-analytic model of halo formation. The
results suggest that numerical simulations of halo formation may still be
missing substructure in the central regions of halos due to purely numerical
effects. If confirmed, this central 'overmerging' problem will have important
consequences for the interpretation of lensing measurements of substructure. We
also show that the spatial distribution of subhalos relative to the satellite
companions of the Milky Way rules out at least one simple model of how dwarf
galaxies form in low-mass halos.
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0704.3337
|
Adam S\'odor
|
The Blazhko behaviour of RR Geminorum II -- long-term photometric
results
|
astro-ph
|
Context. RR Gem is one of the few Blazhko RR Lyrae that has photometric
observations available extended enough to study the long-term courses of its
pulsation and modulation properties in detail.
Aims. We investigate the pulsation and modulation properties and the
relations between them in RR Gem using photometric observations from the past
70 years in order to gain further insight into the nature of the Blazhko
modulation.
Methods. We studied the photographic, photoelectric, and CCD light curves
obtained at the Konkoly Observatory and other authors' published maxima
observations. Detailed analysis of the light curves, maximum brightness, and
O-C data are carried out.
Results. RR Gem showed modulation most of the time it was observed. The
modulation amplitude showed strong variations from the undetectable level (less
than 0.04 mag in maximum brightness) to about 0.20 mag. The amplitudes of the
amplitude and phase modulations showed parallel changes, thus the total 'power'
of the modulation have changed during the past 70 years. Parallel changes in
the pulsation and modulation periods occur with a d P_mod / d P_puls = 1.6 +/-
0.8 * 10^3 ratio. We also detected 0.05-0.1 mag changes in the mean maximum
brightness and mean pulsation amplitude.
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0710.3486
|
Abhik Kumar Sanyal
|
Smooth crossing of $w_{\Lambda} = -1$ line in a single scalar field
model
|
astro-ph
|
Smooth double crossing of the phantom divide line $w_{\Lambda}=-1$ has been
found possible with a single minimally coupled scalar field for the most simple
form of generalized k-essence cosmological model, in the presence of background
cold dark matter. Such crossing is a sufficiently late time transient phenomena
and does not have any pathological behaviour.
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astro-ph/9905052
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Tiziana Di Matteo
|
Hard X-ray emission from elliptical galaxies
|
astro-ph
|
We report the detection of hard X-ray emission components in the spectra of
six nearby, giant elliptical galaxies observed with the ASCA satellite. The
systems studied, which exhibit strong dynamical evidence for supermassive black
holes in their nuclei, are M87, NGC 1399 and NGC 4696 (the dominant galaxies of
the Virgo, Fornax and Centaurus clusters, respectively) and NGC 4472, 4636 and
4649 (three further giant ellipticals in the Virgo cluster). The ASCA data for
all six sources provide clear evidence for hard emission components, which can
be parameterized by power-law models with photon indices in the range \Gamma =
0.6-1.5 (mean value 1.2) and intrinsic 1-10 keV luminosities of 2 \times
10^{40} - 2 \times 10^{42} erg/s. Our results imply the identification of a new
class of accreting X-ray source, with X-ray spectra significantly harder than
those of binary X-ray sources, Seyfert nuclei or low luminosity AGN, and
bolometric luminosities relatively dominated by their X-ray emission. We
discuss various possible origins for the hard X-ray emission and argue that it
is most likely to be due to accretion onto the central supermassive black
holes, via low-radiative efficiency accretion coupled with strong outflows. In
the case of M87, our detected power-law flux is in good agreement with a
previously-reported measurement from ROSAT High Resolution Imager observations,
which were able to resolve the jet from the nuclear X-ray emission components.
We confirm previous results showing that the use of multiphase models in the
analysis of the ASCA data leads to determinations of approximately solar
emission-weighted metallicities for the X-ray gas in the galaxies. We also
present results on the individual element abundances in NGC 4636.
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astro-ph/9807089
|
Durrer Ruth
|
Microwave Background Anisotropies from Alfven waves
|
astro-ph
|
We investigate microwave background anisotropies in the presence of
primordial magnetic fields. We show that a homogeneous field with fixed
direction can amplify vector perturbations. We calculate the correlations of
$\delta T/T$ explicitly and show that a large scale coherent field induces
correlations between $a_{\ell-1,m}$ and $a_{\ell+1,m}$. We discuss constraints
on amplitude and spectrum of a primordial magnetic field imposed by
observations of CMB anisotropies.
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astro-ph/9808335
| null |
Consistency of the metallicity distributions of nearby F, G and K dwarfs
|
astro-ph
|
The consistency of the metallicity distributions of F, G and K dwarfs is
studied. We present a new metallicity distribution for K dwarfs using
metallicities determined from uvby photometry. There is a remarkable paucity of
metal-poor K dwarfs in analogy with the G dwarf problem. We show that late-type
dwarfs have consistent metallicity distributions. We also propose preliminary
corrections to these distributions to take into account the contamination of
the uvby indices due to the chromospheric activity in these stars, since around
30% of the nearby late-type dwarfs have active chromospheres. We consider the
possibility that the metallicity distribution of cooler stars may be different
from that of the hotter stars due to (i) metal-enhanced star formation and (ii)
a metallicity bias in the catalogue of nearby stars. We conclude that these
hypotheses are unlikely to produce important differences in the metallicity
distributions of late-type dwarfs.
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astro-ph/0306406
|
Alexander Lutovinov
|
RXTE observations of X-ray transients IGRJ17091-3624 and IGRJ18539+072
|
astro-ph
|
We present results of analysis of observations of two transient sources IGR
J17091-3624 and IGR J18539+0727 in April 2003 with RXTE observatory. Obtained
energy spectra of sources, and also power spectra of their flux variations give
us a possibility to classify them as X-ray binary systems in low/hard spectral
state. Parameters of power spectrum of IGR J18539+0727 indicates that this
source is a black hole candidate.
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0804.3034
|
Silvia Catalan
|
The initial-final mass relationship of white dwarfs revisited: effect on
the luminosity function and mass distribution
|
astro-ph
|
The initial-final mass relationship connects the mass of a white dwarf with
the mass of its progenitor in the main-sequence. Although this function is of
fundamental importance to several fields in modern astrophysics, it is not well
constrained either from the theoretical or the observational points of view. In
this work we revise the present semi-empirical initial-final mass relationship
by re-evaluating the available data. The distribution obtained from grouping
all our results presents a considerable dispersion, which is larger than the
uncertainties. We have carried out a weighted least-squares linear fit of these
data and a careful analysis to give some clues on the dependence of this
relationship on some parameters such as metallicity or rotation. The
semi-empirical initial-final mass relationship arising from our study covers
the range of initial masses from 1.0 to 6.5 M_sun, including in this way the
low-mass domain, poorly studied until recently. Finally, we have also performed
a test of the initial-final mass relationship by studying its effect on the
luminosity function and on the mass distribution of white dwarfs. This was done
by using different initial-final mass relationships from the literature,
including the expression derived in this work, and comparing the results
obtained with the observational data from the Palomar Green Survey and the
Sloan Digital Sky Survey (SDSS). We find that the semi-empirical initial-final
mass relationship derived here gives results in good agreement with the
observational data, especially in the case of the white dwarf mass
distribution.
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astro-ph/0208257
|
Andrew Benson
|
Galaxy Voids in Cold Dark Matter Universes
|
astro-ph
|
We present predictions for numerous statistics related to the presence of
voids in the distribution of galaxies in a cold dark matter model of structure
formation using a semi-analytic model of galaxy formation. Our study is able to
probe galaxies with masses as low as 10^9Msun/h corresponding to absolute
magnitudes of M_bJ-5log(h)=-18.1 and M_r-5log(h)=-18.7. We quantify the void
and underdense probability functions, distributions of nearest neighbour
distances and void sizes and compute the density profiles of voids. These
results are contrasted with the expectations for dark matter (and the
difference examined in terms of the galaxy/dark matter biasing relation) and
are compared to analytic predictions and observational data where available.
The predicted void probability functions are consistent with those measured
from the Center for Astrophysics redshift surveys given the rather large
uncertainties in this relatively small (for studies of voids) observational
sample. The size of the observational sample is too small to probe the bias
between galaxies and dark matter that we predict. We also examine the predicted
properties of galaxies living within voids and contrast these with the general
galaxy population. Our predictions are aimed at forthcoming large galaxy
redshift surveys which should for the first time provide statistically accurate
measures of the void population.
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astro-ph/9812417
| null |
Cosmological Constraints from the Clustering Properties of the X-Ray
Brightest Abell-type Cluster Sample
|
astro-ph
|
We present an analysis of the 2-point correlation function, of the X-ray
Brightest Abell-type Cluster sample (XBACs; Ebeling et al. 1998) and of the
cosmological constraints that it provides. If \xi(r) is modelled as a
power-law, we find r_0=26.0 +/- 4.5 Mpc/h and \gamma=2.0 +/-0.4, with errors
corresponding to 2\sigma uncertainties. Only a marginal increase of the
correlation amplitude is found as the flux limit is increased from 5x10^{-12}
cgs to 12x10^{-12} cgs, thus indicating a weak dependence of the correlation
amplitude on the cluster X-ray luminosity. Furthermore, we present a method to
predict correlation functions for flux-limited X-ray cluster samples from
cosmological models. The method is based on the analytical recipe by Mo & White
(1996) and on an empirical approach to convert cluster fluxes into masses. We
use a maximum-likelihood method to place constraints on the model parameter
space. We find that the shape parameter of the power spectrum lies in the
2\sigma range 0.05<\Gamma<0.20. As for the amplitude of the power-spectrum, we
find \sigma_8 in the range 0.4-0.8 for \Omega_0=1 and 0.8-2.0 for \Omega_0=0.3.
This result is in agreement with, although less constraining than, results
based on the local cluster abundance.
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astro-ph/0403311
|
Sandra Etoka
|
OH spectral evolution of oxygen-rich late-type stars
|
astro-ph
|
We investigated the main-line spectral evolution with shell thickness of
oxygen rich AGB stars. The study is based on a sample of 30 sources distributed
along the IRAS colour-colour diagram. The sources were chosen to trace the
Miras with thick shells and the whole range of OH/IR stars. The Miras exhibit a
1665 MHz emission strength comparable to that at 1667 MHz. Even though the
Miras of the study have quite thick shells, their spectral characteristics in
both main lines attest to a strong heterogeneity in their OH shell with, in
particular, the presence of significant turbulence and acceleration. The
expansion velocity has been found to be about the same at 1665 and 1667 MHz,
taking into account a possible velocity turbulence of 1-2km/s at the location
of the main-line maser emission. An increase in the intensity ratio 1667/1665
with shell thickness has been found. A plausible explanation for such a
phenomenon is that competitive gain in favour of the 1667 MHz line increases
when the shell is getting thicker. There is an evolution in the spectral
profile shape with the appearance of a substantial inter-peak signal when the
shell is getting thicker. Also, inter-peak components are found and can be as
strong as the external standard peaks when the shell is very thick. This trend
for an increase of the signal in between the two main peaks is thought to be
the result of an increase of the saturation with shell thickness. All sources
but two - a Mira and an OH/IR star from the lower part of the colour-colour
diagram - are weakly polarized. The strong polarization observed for those two
particular objects is thought to be the result of perturbations in their
shells.
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astro-ph/0409147
|
Huib Jan van Langevelde
|
Molecular absorption in Cen A on VLBI scales
|
astro-ph
|
Centaurus A, the nearest AGN shows molecular absorption in the millimeter and
radio regime. By observing the absorption with VLBI, we try to constrain the
distribution of the gas, in particular whether it resides in the circumnuclear
region. Analysis of VLBA observations in four OH and two H2CO transitions is
presented here, as well as molecular excitation models parameterized with
distance from the AGN. We conclude that the gas is most likely associated with
the tilted molecular ring structure observed before in molecular emission and
IR continuum. The formaldehyde absorption shows small scale absorption which
requires a different distribution than the hydroxyl.
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0704.0973
|
Fangjun Lu
|
X-ray Timing Observations of PSR J1930+1852 in the Crab-like SNR
G54.1+0.3
|
astro-ph
|
We present new X-ray timing and spectral observations of PSR J1930+1852, the
young energetic pulsar at the center of the non-thermal supernova remnant
G54.1+0.3. Using data obtained with the Rossi X-ray Timing Explorer and Chandra
X-ray observatories we have derived an updated timing ephemeris of the 136 ms
pulsar spanning 6 years. During this interval, however, the period evolution
shows significant variability from the best fit constant spin-down rate of
$\dot P = 7.5112(6) \times 10^{-13}$ s s$^{-1}$, suggesting strong timing noise
and/or glitch activity. The X-ray emission is highly pulsed ($71\pm5%$
modulation) and is characterized by an asymmetric, broad profile ($\sim 70%$
duty cycle) which is nearly twice the radio width. The spectrum of the pulsed
emission is well fitted with an absorbed power law of photon index $\Gamma =
1.2\pm0.2$; this is marginally harder than that of the unpulsed component. The
total 2-10 keV flux of the pulsar is $1.7 \times 10^{-12}$ erg cm$^{-2}$
s$^{-1}$. These results confirm PSR J1930+1852 as a typical Crab-like pulsar.
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astro-ph/9903314
|
Mark Lacy
|
Radio-optical alignments in a low radio luminosity sample
|
astro-ph
|
We present an optically-based study of the alignment between the radio axes
and the optical major axes of eight z~0.7 radio galaxies in a 7C sample. The
radio galaxies in this sample are ~20-times less radio luminous than 3C
galaxies at the same redshift, and are significantly less radio-luminous than
any other well-defined samples studied to date. Using Nordic Optical Telescope
images taken in good seeing conditions at rest-frame wavelengths just longward
of the 4000A break, we find a statistically significant alignment effect in the
7C sample. Furthermore, in two cases where the aligned components are well
separated from the host we have been able to confirm spectroscopically that
they are indeed at the same redshift as the radio galaxy. However, a
quantitative analysis of the alignment in this sample and in a corresponding 3C
sample from HST archival data indicates that the percentage of aligned flux may
be lower and of smaller spatial scale in the 7C sample. Our study suggests that
alignments on the 50-kpc scale are probably closely related to the radio
luminosity, whereas those on the 15 kpc scale are not. We discuss these results
in the context of popular models for the alignment effect.
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astro-ph/0002467
|
Ravi Subrahmanyan
|
An Australia telescope survey for CMB anisotropies
|
astro-ph
|
We have surveyed six distinct `empty fields' using the Australia Telescope
Compact Array in an ultra-compact configuration with the aim of imaging, with a
high brightness sensitivity, any arcmin-scale brightness-temperature
anisotropies in the background radio sky. The six well-separated regions were
observed at a frequency of 8.7 GHz and the survey regions were limited by the
ATCA primary beams which have a full width at half maximum of 6 arcmin at this
frequency; all fields were observed with a resolution of 2 arcmin and an rms
thermal noise of 24 microJy/beam. After subtracting foreground confusion
detected in higher resolution images of the fields, residual fluctuations in
Stokes I images are consistent with the expectations from thermal noise and
weaker (unidentified) foreground sources; the Stokes Q and U images are
consistent with expectations from thermal noise.
Within the sensitivity of our observations, we have no reason to believe that
there are any Sunyaev-Zeldovich holes in the microwave sky surveyed. Assuming
Gaussian-form CMB anisotropy with a `flat' spectrum, we derive 95 per cent
confidence upper limits of Q_flat < 10--11 microK in polarized intensity and
Q_flat < 25 microK in total intensity. The ATCA filter function peaks at l=4700
and has half maximum values at l=3350 and 6050.
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