Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Probing the stellar wind environment of Vela X-1 with MAXI   Vela X-1 is among the best studied and most luminous accreting X-ray pulsars.
The supergiant optical companion produces a strong radiatively-driven stellar
wind, which is accreted onto the neutron star producing highly variable X-ray
emission. A complex phenomenology, due to both gravitational and radiative
effects, needs to be taken into account in order to reproduce orbital spectral
variations. We have investigated the spectral and light curve properties of the
X-ray emission from Vela X-1 along the binary orbit. These studies allow to
constrain the stellar wind properties and its perturbations induced by the
compact object. We took advantage of the All Sky Monitor MAXI/GSC data to
analyze Vela X-1 spectra and light curves. By studying the orbital profiles in
the $4-10$ and $10-20$ keV energy bands, we extracted a sample of orbital light
curves (${\sim}15$% of the total) showing a dip around the inferior
conjunction, i.e., a double-peaked shape. We analyzed orbital phase-averaged
and phase-resolved spectra of both the double-peaked and the standard sample.
The dip in the double-peaked sample needs $N_H\sim2\times10^{24}\,$cm$^{-2}$ to
be explained by absorption solely, which is not observed in our analysis. We
show how Thomson scattering from an extended and ionized accretion wake can
contribute to the observed dip. Fitted by a cutoff power-law model, the two
analyzed samples show orbital modulation of the photon index, hardening by
${\sim}0.3$ around the inferior conjunction, compared to earlier and later
phases, hinting a likely inadequacy of this model. On the contrary, including a
partial covering component at certain orbital phase bins allows a constant
photon index along the orbital phases, indicating a highly inhomogeneous
environment. We discuss our results in the framework of possible scenarios.

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The categories of this paper are: nucl-ex
2
The categories of this paper are: math
13
The categories of this paper are: quant-ph
16
The categories of this paper are: stat
5
The categories of this paper are: gr-qc
32
The categories of this paper are: atom-ph
33
The categories of this paper are: plasm-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-lat
3
The categories of this paper are: physics
36
The categories of this paper are: cmp-lg
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The categories of this paper are: q-fin
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The categories of this paper are: dg-ga
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The categories of this paper are: hep-th
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The categories of this paper are: hep-ex
0
The categories of this paper are: hep-ph