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 |