Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: High-energy gamma-ray emission from Cyg X-1 measured by Fermi and its
  theoretical implications   We have obtained measurements and upper limits on the emission of Cyg X-1 in
the photon energy range of 0.03--300 GeV based on observations by Fermi. We
present the results separately for the hard and soft spectral states, as well
for all of the analysed data. In the hard state, we detect a weak steady
emission in the 0.1--10 GeV range with a power-law photon index of
Gamma=2.6+-0.2 at a 4 sigma statistical significance. This measurement, even if
considered to be an upper limit, strongly constrains Compton emission of the
steady radio jet, present in that state. The number of relativistic electrons
in the jet has to be low enough for the spectral components due to Compton
upscattering of the stellar blackbody and synchrotron radiation to be within
the observed fluxes. If optically-thin synchrotron emission of the jet is to
account for the MeV tail, as implied by the recently-claimed strong
polarization in that energy range, the magnetic field in the jet has to be much
above equipartition. The GeV-range measurements also strongly constrain models
of hot accretion flows, most likely present in the hard state, in which
gamma-rays are produced from decay of neutral pions produced in collisions of
energetic ions in an inner part of the flow. In the soft state, the obtained
upper limits constrain electron acceleration in a non-thermal corona, most
likely present around a blackbody accretion disc. The coronal emission above 30
MeV has to be rather weak, which is most readily explained by absorption of
gamma-rays in pair-producing photon-photon collisions. Then, the size of the
bulk of the corona is less than a few tens of the gravitational radii.

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The categories of this paper are: hep-ex
25
The categories of this paper are: patt-sol
33
The categories of this paper are: plasm-ph
30
The categories of this paper are: q-alg
21
The categories of this paper are: adap-org
29
The categories of this paper are: funct-an
2
The categories of this paper are: math
16
The categories of this paper are: stat
17
The categories of this paper are: q-fin
14
The categories of this paper are: math-ph
22
The categories of this paper are: chao-dyn
34
The categories of this paper are: chem-ph
26
The categories of this paper are: bayes-an
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The categories of this paper are: hep-th
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The categories of this paper are: nucl-th
32
The categories of this paper are: atom-ph