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Astron. Astrophys. 354, 983-986 (2000)

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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, NHe[FORMULA]4[FORMULA]1023 cm-2 [in this calculation, we have taken the mass loss [FORMULA]=4[FORMULA]10-5 [FORMULA] yr -1 (van Kerkwijk 1993); the wind velocity law is taken as v(r)= va [FORMULA] with [FORMULA][FORMULA]1.0 and va=4000 km s-1 (Conti and Underhill 1988)] This value will correspond to a hydrogen column density of [FORMULA]4[FORMULA]1024 cm-2 (corresponding to solar abundances); this is much larger than the observed column density of 7[FORMULA]1022 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 nH (Universal abundances), X-radiation of luminosity L, will ionise a sphere of radius R given by R2=L/nH[FORMULA]. Kallman and McCray (1982) give [FORMULA][FORMULA]100 for the ionisation described above.With L[FORMULA]1038 erg s-1 and nH=7[FORMULA]1013 cm-3 (corresponding to nHe=7[FORMULA]1013 cm-3), we find R[FORMULA]1.2[FORMULA]1011 cm, giving a column density of [FORMULA]8.4 [FORMULA]1024H 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 [FORMULA]/2 times the value at phase 0.75 (see Williams et al. 1990). The Compton scattering optical depth due to the Wolf-Rayet wind [FORMULA] [FORMULA]2NHe[FORMULA], where [FORMULA] is the Compton scattering cross section. At phase 0.75, [FORMULA] is about 0.6 and at a phase 0.5 it is [FORMULA]/2 times this value, that is [FORMULA]0.9.

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© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000