6.4. Galactic Black Hole Candidates

Smeared edges with little evidence of line emission have been observed in the spectra of Galactic Black Hole Candidates (GBHC) (Ebisawa et al 1996). These observations can be explained if the surface of the disk is moderately ionized with a mean ionization parameter of a few hundred (Fig. 2 with = 100-300; Ross, Fabian & Brandt 1996). As discussed in Section 2.1, for this relatively narrow range of ionization parameters line photons are resonantly trapped and eventually lost as Auger electrons. Hence, the X-ray reflection produces very little line emission but an appreciable absorption edge. Sharp features will be smeared as a result of the Doppler and relativistic effects, and this blurring has possibly been detected in the spectrum of Cygnus X-1 (Done & Zycki 1999). Similar spectra but with broader emission and absorption features are produced for higher values of the ionization parameter 3 x 10³, which appear to match those seen in GBHC. The smearing in this instance is due to the line photons being generated a few Thomson scattering depths into the disk (the very outermost layers are completely ionized) and being Compton scattered on leaving it.

An interesting correlation has been claimed by Zdziarski, Lubinski & Smith (1999) between the reflection fraction seen in accreting black holes (AGN or GBHC) and the spectra index. Sources with flat spectra tend to have a low reflection fraction. Models involving large central holes in the disc, or ionized discs, may explain the correlation, as may mild relativistic motion, thus beaming, of the continuum radiation (Beloborodov 1999; see also Reynolds & Fabian 1997).

It is currently difficult to discriminate between models in which a cold disk truncates at a few tens of Schwarzschild radii (e.g. Gierlinski et al 1997) and models in which an ionized disk extend in to the innermost stable orbit (Young et al 2000) since both provide good fits to present data. Future observations will hopefully resolve these issues.