Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the origin of the hard X-ray tail in neutron-star X-ray binaries   Neutron star X-ray binaries emit a compact, optically thick, relativistic
radio jet during low-luminosity, usually hard states, as Galactic black-hole
X-ray binaries do. When radio emission is bright, a hard power-law tail without
evidence for an exponential cutoff is observed in most systems. We have
developed a jet model that explains many spectral and timing properties of
black-hole binaries in the states where a jet is present. Our goal is to
investigate whether our jet model can reproduce the hard tail, with the correct
range of photon index and the absence of a high-energy cutoff, in neutron-star
X-ray binaries. We have performed Monte Carlo simulations of the Compton
upscattering of soft, accretion-disk or boundary layer photons, in the jet and
computed the emergent energy spectra, as well as the time lag of hard photons
with respect to softer ones as a function of Fourier frequency. We demonstrate
that our jet model explains the observed power-law distribution with photon
index in the range 1.8-3. With an appropriate choice of the parameters, the
cutoff expected from Comptonization is shifted to energies above ~300 keV,
producing a pure power law without any evidence for a rollover, in agreement
with the observations. Our results reinforce the idea that the link between the
outflow (jet) and inflow (disk) in X-ray binaries does not depend on the nature
of the compact object, but on the process of accretion. Furthermore, we address
the differences of jets in black-hole and neutron-star X-ray binaries and
predict that the break frequency in the spectral energy distribution of
neutron-star X-ray binaries, as a class, will be lower than that of black-hole
binaries.

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The categories of this paper are: hep-ph
5
The categories of this paper are: gr-qc
35
The categories of this paper are: mtrl-th
23
The categories of this paper are: solv-int
12
The categories of this paper are: nucl-th
30
The categories of this paper are: q-alg
27
The categories of this paper are: comp-gas
28
The categories of this paper are: alg-geom
18
The categories of this paper are: econ
21
The categories of this paper are: adap-org
33
The categories of this paper are: plasm-ph
11
The categories of this paper are: hep-lat
17
The categories of this paper are: q-fin
32
The categories of this paper are: atom-ph
26
The categories of this paper are: bayes-an
37
The categories of this paper are: supr-con