LINE-PROFILES FROM A DISK AROUND A ROTATING BLACK-HOLE

The iron K-alpha-line is detected in Galactic X-ray sources and in some active galactic nuclei (AGNs). This line is likely to be produced by fluorescence from a cold X-ray-irradiated gas. The line is probably resolved in few cases, and shows a width of about 1 keV. This width is naturally explained by Doppler broadening due to the motion of the line-emitting gas in a thin Keplerian accretion disk. Previous studies of line profiles have concentrated on the case of a disk around a nonrotating black hole. The black hole is, however, very likely to be rotating, in which case the disk extends to a smaller radius, where the relativistic effects become stronger. The inner part of the disk is especially relevant to X-ray line emission, since the short variability time scales of the continuum source suggest that most of the emission comes from a small region, possibly close to the center of the disk. This paper presents calculations of profiles of lines emitted from an accretion disk around a rotating black hole. The calculations were done for a number of different line emissivity laws, with an emphasis on cases where most of the flux originates from the inner regions of the disk. The main results are as follows: (1) Most emissivity laws produce a broad single-peaked line. A double-peaked profile generally characterizes emission from the outer parts of the accretion disk. (2) The line is significantly asymmetric in most cases, having a steep blue wing and an extended red wing. (3) The line peak is blueshifted in most cases. In a few of these cases the blue wing of the line may extend up to an energy shift of 50%. A redshifted line is produced only by a nearly face-on disk, and the maximum redshift amounts to less than 5%. The strong relativistic effects, in particular a blueshift of more than 30%, are produced only in the innermost regions of the accretion disk, and are therefore unique to the case of a rotating black hole. These effects can be strong enough to be reflected even in the low-resolution data currently available. The available observations, however, do not show these effects, and this implies that most of the observed line is generated outside the innermost disk. Future higher resolution spectroscopic data will place better constraints on the possible contribution to the line emission from various parts of the disk, and possibly test the very existence of thin accretion disks in these systems.