Reflection of X-rays off companion stars and accretion disks
When X-rays are reflected off of surfaces such as the atmospheres of
stars and accretion disks we call the effect "Compton reflection".
The effect is common to a large number of X-ray sources, most-notably
active galactic nuclei and galactic black holes. Although models of
reflection are used widely as a diagnostic of accreting sources, they
remain poorly calibrated against well-understood atmospheric
environments. There are question marks over the location and extent
of the central X-ray source, and the incidence angles of the X-rays
that illuminate the reflecting atmosphere. Since both direct and
reflected photons are observed together, the reflecting source is
typically a few percent of the overall flux. Consequently,
contamination from the brighter spectral component handicaps our
attempts to characterize and quantify reflection.
The detection of an almost uncontaminated reflection spectrum is a
rare event, but important in order to test and calibrate our
understanding of X-ray reflection. One such source is the the
companion star of the eclipsing X-ray pulsar Her~X-1 which is only
visible in X-rays when the pulsar is obscured from our line-of-sight
by a warped accretion disk. This environment is well-understood, we
know the companion star's mass, size, shape, intrinsic temperature,
composition and distance from the X-ray source. Atmospheric
parameters such as density and pressure are well-constrained from
main-sequence tables. The size, extent, spectrum and intensity of the
pulsed X-ray source are also known. The characteristic light curve of
X-rays reflected off the hemisphere of the companion star facing the
pulsar is presented in the figure. Maximum light occurs when the
companion star is directly behind the pulsar.
Also below are spectral fits to three of the visits to Her X-1. The top
left panel is a powerlaw + Gaussian profile fit to mid-eclipse data (MJD
51371.98). The right hand panel presents the best fit model. The middle
two panels are the corresponding fit and model for the post-eclipse visit
at MJD 51372.15. At this time we cannot see the reflecting surface, which
is over the limb of the companion star. The observed flux is light from
the pulsar (a powerlaw continuum plus broad iron emission line), scattered
off a corona over the accretion disk. The lower panels represent the visit
at maximum flux, MJD 51372.90. Here we combine the coronal spectrum with a
reflection spectrum comprising hard continuum component and a narrow iron
fluorescence line at 6.4 keV.
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