*Astron. Astrophys. 354, 983-986 (2000)*
## 4. Scattering and absorption of X-rays from the black-hole by the post-shock gas
The determination of the scattering or absorption optical depth
will depend on the structure of the gas density, starting from the
shock front to the black hole. Since we have not obtained this density
profile, we will estimate the electron scattering optical depth in two
steps; 1) the electron scattering optical depth
from the profiles given in
Sect. (2) and the optical depth
due to the gas ionised and controlled by the X-ray irradiation. The
electron column density N_{c} between the black hole
and an observer seeing it from the front of the shock due to the hot
post-shock gas can be calculated using the density and temperature
profiles given above and is N_{c}=32
n_{0}(/a). We find
N_{c}4.2
10^{23} cm^{-2}. This corresponds to an electron
scattering optical depth of
0.3.
We have also calculated from the
black hole to the observer in a direction perpendicular to the motion
of the shock and find it is about the same value. In the Appendix the
optical depth for Compton scattering due to the gas ionised by the
X-radiation from the black hole ,
when an observer sees the black hole from the front of the shock, is
given for various n_{He}. For helium gas densities
greater than 10^{15} cm^{-3} the
is large and will scatter and
degrade the X-radiation. Since the hard radiation is observed at a
high intensity the scattering optical depth
must be
1; we will estimate this below. The
value of in the direction
perpendicular to the motion of the shock at the black hole will be
twice the value of that seen from the front, if the shock front is
assumed to be in the shape of a parabola. Thus flux of X-radiation
from the black hole as seen from the side of the shock will be less
than that seen from the front of the shock. The thick accretion wake
is however dense enough to attenuate both components when the observer
views the black hole from the back of the shock, that is at phase
0.0.
© European Southern Observatory (ESO) 2000
Online publication: February 25, 2000
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