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*Astron. Astrophys. 354, 983-986 (2000)*
## 5. Absorption and scattering of X-radiation by the wind of the Wolf-Rayet star
The X-radiation emitted by the black hole as well as that emitted
by the post-shock gas has to pass through the wind of the Wolf-Rayet
star. A simple estimate of the column density of helium from the
position of the compact star at phase 0.75 to the observer (see
Fig. 1) gives,
N_{He}410^{23}
cm^{-2} [in this calculation, we have taken the mass loss
=410^{-5}
yr ^{-1} (van Kerkwijk
1993); the wind velocity law is taken as v(r)= v_{a}
with
1.0
and v_{a}=4000 km s^{-1} (Conti and Underhill
1988)] This value will correspond to a hydrogen column density of
410^{24}
cm^{-2} (corresponding to solar abundances); this is much
larger than the observed column density of
710^{22} H atoms
cm^{-2} (White and Holt 1982; Willingale et al. 1985). However
the strong X-ray emission from the compact object and the hot
post-shock gas ionises the gas around(Kallman and McCray 1982); the
ionisation is complete upto the element Silicon and partial upto the
element Iron. In a gas of uniform hydrogen number density
n_{H} (Universal abundances), X-radiation of luminosity
L, will ionise a sphere of radius R given by
R^{2}=L/n_{H}.
Kallman and McCray (1982) give
100
for the ionisation described above.With
L10^{38} erg s^{-1}
and
n_{H}=710^{13}
cm^{-3} (corresponding to
n_{He}=710^{13}
cm^{-3}), we find
R1.210^{11}
cm, giving a column density of 8.4
10^{24}H atoms
cm^{-2}. In actuality at phase 0.75, the wind density is
falling towards the observer giving a lower column density. However
comparing this number with the column density estimate due to the
wind, suggests that most of the wind in this direction is completely
ionised upto at least Silicon and partially ionised upto Iron. Thus
the soft X-radiation is not appreciably absorbed by the wind and
results in the low hydrogen column density observed. The column
density from the compact object to the observer due to the wind at
phase 0.5 can also be calculated and is
/2 times the value at phase 0.75 (see
Williams et al. 1990). The Compton scattering optical depth due to the
Wolf-Rayet wind
2N_{He},
where is the Compton scattering
cross section. At phase 0.75, is
about 0.6 and at a phase 0.5 it is /2
times this value, that is 0.9.
© European Southern Observatory (ESO) 2000
Online publication: February 25, 2000
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