The past few years have seen a shift in the mind-set of many observationally-oriented black hole researchers. A small number of important observations (the mass of the compact object in the binary star system Cyg X-1, stellar motions in our Galactic Center, the gas disk in M87, and the maser disk in NGC 4258) have led most astronomers to the conclusions that black holes do indeed exist beyond any reasonable doubt. With this established, the interest has shifted to the demographics of black holes in the Universe, and the detailed astrophysics of black hole systems.
As we have described, X-ray spectroscopy currently provides the best
understood method of exploring the astrophysical environment in the
immediate vicinity of an accreting black hole. The accretion disk is
the engine that drives all observable phenomena from accreting black
hole systems. In many systems, the surface layers of the accretion
disk are expected to produce X-ray spectral features (so-called X-ray
reflection signatures) in response to external hard X-ray illumination
from a disk-corona -- the most prominent spectral feature is often
the fluorescent K
emission line of iron. This line, which has
an intrinsically small energy width, will be dramatically broadened
and skewed by the rapid orbital motions of the accreting material and
strong gravity of the black hole. These relativistic spectral
features can be identified unambiguously in many of the AGN for
which X-ray data of sufficient quality exist. Similar spectral
features can also be found in many AGN and GBHCs for which poorer
quality data exist -- while it may not be possible to rule out other
forms of spectral complexity in these objects, relativistic disk
features are often the most compelling and physical of the
possibilities. Given how generic these features are, they provide a
powerful way to study the near environment of accreting black holes.
Moderate luminosity radio-quiet AGN, the Seyfert galaxies, present the cleanest examples of these spectral features. Analysis of X-ray data from ASCA and, more recently, XMM-Newton finds evidence for a rather cold accretion disk extending all of the way down to the radius of marginal stability around the black hole. In one case (MCG-6-30-15), the data argue strongly that the black hole is rapidly-rotating. Furthermore, there may be suggestions that the accretion disk is torqued by processes associated with the spinning black hole and, in particular, may be tapping into the rotational energy of the spinning black hole. The case of MCG-6-30-15, which we discussed in some detail, illustrates the exotic black hole physics that can be addressed via these techniques.
The difference in iron line properties between Seyfert galaxies and other types of AGN allows us to probe the physical state of the accretion flow as a function of, in particular, the mass accretion rate. The fact that higher-luminosity AGN display weaker spectral signatures is very likely due to increasing ionization of the disk surface as the accretion rate (relative to that rate which would produce the Eddington luminosity) is increased. The weakness of the X-ray reflection displayed by radio-loud AGN and low-luminosity AGN is much less certain (primarily due to the paucity of high quality datasets). Possibilities include ionization of the disk surface, dilution of the disk spectrum by X-ray emission from a relativistic jet, and/or the transition of the disk into a radiatively-inefficient state (which would be extremely hot and optically-thin thereby producing no X-ray reflection features).
Although it is somewhat counter-intuitive given that they can be three orders of magnitude brighter than AGN, it has been much more difficult to study relativistic disk signatures in GBHCs. In addition to the fact that the X-ray spectra of GBHC are inherently more complex (since the X-ray band contains the thermal disk emission and the disk is almost always strongly ionized), the high X-ray fluxes from these sources can readily overwhelm sensitive photon counting spectrometers. However, with Chandra and XMM-Newton we can now study relativistic X-ray reflection in GBHCs free of these instrumental constraints, at medium-to-high spectral resolution, and high signal-to-noise. Consequently, relativistic iron lines have been found in several GBHCs with a variety of ``spectral states''. With further study of these features over the coming few years, we should be able to constrain the gross geometry of the accretion flow as a function of spectral state.
The future is bright. Until just a few years ago, the physics of relativistic accretion disks and the astrophysics of black hole spin was firmly in the realm of pure theory. Modern X-ray spectroscopy has given us a powerful tool with which we can peer into this exotic world. There is still much to be learnt -- much of the physics determining the properties of accretion disks, the demographics of black hole spin (which is closely related to black hole formation and history), and the prevalence of spin-energy extraction (which is of fundamental interest) are just a few of the open questions. Continued investigation with increasingly capable instruments promise to answer these, and many more, questions.