Lastly, one additional piece of evidence, although it does not give a direct measure of the central mass, cannot go unmentioned - namely the recent detection in AGNs of the broad iron K line at 6.4 keV. This line has been known for some time to be a common feature in the hard X-ray spectra of AGNs, and it is thought to arise from fluorescence of the X-ray continuum off of cold material, presumably associated with the accretion disk around the SMBH (e.g., Pounds et al. 1990). The spectral resolution of the existing data, however, was insufficient to test the predicted line profile (Fabian et al. 1989). The ASCA satellite provided the much-awaited tell-tale signs in the deep exposure of the Seyfert 1 galaxy MCG-6-30-15 (Tanaka et al. 1995): the Fe K line exhibits Doppler motions that approach relativistic speeds (~ 100,000 km s-1 or 0.3c!) as well as an asymmetric red wing consistent with gravitational redshift. The best-fitting disk has an inner radius of 6 Schwarzschild radii. The relativistic Fe K line, now seen in a large number of sources (Nandra et al. 1997; Fig. 7), provides arguably the most compelling evidence to date for the existence of SMBHs. Other mechanisms for generating the line profile are possible, but implausible (Fabian et al. 1995). Detailed modeling of the line asymmetry has even the potential to measure the spin of the hole, but this is still very much a goal of the future given the current data quality and uncertainties in the modeling itself (e.g., Reynolds & Begelman 1997; Rybicki & Bromley 1998).
Figure 7.The Fe K line in the composite spectrum of Seyfert 1 nuclei (Nandra et al. 1997). The solid line is a fit to the line profile using two Gaussians, a narrow component centered at 6.4 keV and a much broader, redshifted component.