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 Nc 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 Nc=32 n0(/a). We find Nc4.2 1023 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 nHe. For helium gas densities greater than 1015 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