Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Continuum, cyclotron line, and absorption variability in the high-mass
  X-ray binary Vela X-1   Because of its complex clumpy wind, prominent cyclotron resonant scattering
features, intrinsic variability and convenient physical parameters (close
distance, high inclination, small orbital separation) which facilitate the
observation and analysis of the system, Vela X-1 is one of the key systems to
understand accretion processes in high-mass X-ray binaries on all scales. We
revisit Vela X-1 with two new observations taken with NuSTAR at orbital phases
~0.68-0.78 and ~0.36-0.52 which show a plethora of variability and allow us to
study the accretion geometry and stellar wind properties of the system. We
follow the evolution of spectral parameters down to the pulse period time-scale
using a partially covered powerlaw continuum with a Fermi-Dirac cut-off to
model the continuum and local absorption. We could confirm anti-correlations
between the photon index and the luminosity and, for low fluxes, between the
folding energy and the luminosity, implying a change of properties in the
Comptonising plasma. We could not confirm a previously seen correlation between
the cyclotron line energy and the luminosity of the source in the overall
observation, but we observed a drop in the cyclotron line energy following a
strong flare. We see strong variability in absorption between the two
observations and within one observation (for the ~0.36-0.52 orbital phases)
that can be explained by the presence of a large-scale structure, such as
accretion- and photoionisation wakes in the system and our variable line of
sight through this structure.

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The categories of this paper are: supr-con
18
The categories of this paper are: econ
25
The categories of this paper are: patt-sol
13
The categories of this paper are: quant-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: adap-org
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The categories of this paper are: cs
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The categories of this paper are: cmp-lg
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The categories of this paper are: physics
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The categories of this paper are: acc-phys
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The categories of this paper are: bayes-an
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The categories of this paper are: math
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The categories of this paper are: alg-geom
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The categories of this paper are: funct-an
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The categories of this paper are: hep-th
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The categories of this paper are: q-fin