3.4. Fe K Emission Line and the Lyman Edge
The Fe K emission line, interpreted as arising from fluorescence in the X-ray-illuminated disk, is extremely broad and redshifted, which provides dramatic support for an accretion disk in a relativistic potential well (Tanaka et al. 1995). It is perhaps the best evidence for accretion disks in the innermost regions of the central engines of AGNs. It certainly provides the only direct evidence for cold gas with a large column density very close to the central engine. It is therefore important to investigate the nature of the Lyman edge feature in objects that have such convincing evidence of an accretion disk. Unfortunately, there are no observations spanning a reasonable wavelength region (see Section 3.2) around the Lyman limit for an AGN that also shows a broad Fe K line. The best data come from the Hopkins Ultraviolet Telescope (HUT) on low-redshift Seyfert galaxies (Kriss et al. 1997), and ASCA observations of these Seyfert galaxies. There are only six Seyfert galaxies that have both Lyman edge and Fe K line observations. Except for perhaps one object (NGC 3516), none of the remaining five objects shows a clear intrinsic Lyman edge feature in total flux. Confusion with Galactic absorption in high-n Lyman series lines and the Lyman continuum preclude using the HUT data to search for the smeared edge features expected from a relativistic disk.
Observations of the Fe K emission line and the Lyman limit region can be used to constrain the theoretical accretion disk models. The Fe K line profiles and line peaks are dependent on the inclination angle of the accretion disk. Although the black hole mass and geometry influence the line profile shape, in general the models predict that the line profiles are broader and the line peaks are bluer for edge-on disks than for face-on disks (see Fig. 12). Therefore, if the line profile is accurately determined, it places strict constraints on the disk inclination angle. Next, if one assumes that the Lyman edge is also from the same disk structure (which need not be the case), then the predicted Lyman edge can be compared with observations (see Figs. 12 and 13). The current Fe K emission-line profiles exclude edge-on disks whose Lyman edges are easier to hide owing to large relativistic smearing. The disk inclination angles derived from the Fe K emission-line profiles indicate that in a "bare" disk (i.e., no Comptonization), the Lyman edge should be detectable. The current data therefore once again suggest that Comptonization is important.
Figure 12. Fe K line profiles for accretion disks around a Schwarzschild black hole at different inclination angles i (µ cos i). The line is assumed to be emitted locally as nearly a delta function in energy at 6.4 keV. A line emissivity profile f(r) which falls off as 1/r with radius r has been assumed, extending from the innermost stable orbit to 14.8 RG. Lines are broader for edge-on disks than for face-on disks. This figure should be compared with Figs. 13 and 19, which show predictions for Lyman edges at similar inclination angles.
Figure 13. Predicted spectral energy distribution in the Lyman limit region for accretion disks around a Schwarzschild black hole at different inclination angles i (µ cos i). Spectral features are much more visible for face-on disks (cos i = 0.98) than for edge-on disks. Here the accretion disk is considered to be made of pure hydrogen and helium. Non-LTE effects and relativistics transfer function have been fully incorporated. External illumination of the disk by X-rays has been neglected, which may increase the prominence of the Lyman edges (cf. Fig. 19).
The Lyman edge problem indicates that to understand the emission mechanism in AGNs, multiwavelength data are essential to constrain the models. With FUSE coming on-line, in addition to high-energy resolution X-ray telescopes such as the X-ray Multi-Mirror Observatory (XMM), there will soon be an opportunity to investigate the Fe K emission line and the Lyman edge simultaneously.