7. SUMMARY

In this section we summarize some of the key points of this review.

• X-ray continuum. The hard X-ray continuum in AGN and GBHCs is thought to be produced in active or flaring regions in a corona above the accretion disk. Thermal electrons multiply inverse Compton scatter optical and UV photons from the disk to X-ray energies. The hard X-ray power law that results irradiates the accretion disk and produces a ``reflection'' component in the spectrum.

• Reflection component. The reflection component causes the observed spectrum to flatten above 10 keV as Compton recoil reduces the backscattered flux and also results in a strong iron fluorescence line at approximately 6.4 keV. The precise energy and strength of the line depend on a number of factors such as the iron abundance, the inclination of the disk and its ionization state. An ionized disk may also produce a strong iron absorption edge.

• Iron line profile. The line profile is determined by Doppler shifts and relativistic boosting due to the motion of the disk and the gravitational redshifting of the black hole. This produces a broad, skewed line profile. Since the line originates from the innermost regions of the accretion disk, these effects are very pronounced. From observations of the line profile the black hole spin and the inclination angle of the accretion disk may be determined. In most Seyfert 1 galaxies (i.e. AGN in which we can view the accreting black hole directly) the accretion disk is inclined at about 30° to the observer. This is consistent with the standard model in which the Seyfert nucleus is surrounded by an optically thick torus with an opening angle of 30-40°.

• Observations. The iron line was first clearly detected by Ginga and a line profile subsequently resolved by ASCA confirming the broad and skewed shape expected from an accretion disk around a Schwarzschild black hole. RXTE has been able to study the line and continuum variability on much shorter timescales, although with reduced energy resolution.

• Black hole spin. The radius of the smallest stable circular orbit around the black hole decreases with the spin of the black hole. Since the line profile is sensitive to the innermost radius of fluorescent emission this may be used (with some assumptions about the astrophysics of this region) to estimate the spin of the black hole. With present time-averaged observations, however, such measurements may be ambiguous as alternative models with very different values for the black hole spin may produce almost identical line profiles.

• Alternative models for the production of the broad iron line. Models for the broad iron line that do not require a black hole accretion disk appear to fail. In particular the line width cannot be entirely due to Comptonization. Hybrid models in which both Comptonization and Doppler/gravitational effects produce the line profile are heavily constrained.

• Variability. Rapid X-ray continuum variability is observed in most AGN and the iron line is expected to vary in response to this with a short time lag. Whilst these timescales are too short to be probed with present instruments, significant and complex iron line variability has been observed. Curiously, the line flux is seen to remain constant whilst the continuum changes, and there appears to be an anti-correlation between the reflected fraction and the equivalent width of the line. In another study the reflected fraction and the photon index of the power law are correlated, both for an individual object, and between different objects (including both AGN and GBHC). Such observations need to be explained, especially since they appear contrary to our simple model of reflection. Flux-correlated changes in the ionization state of the disk may explain some of these facts.

• Reverberation mapping. The rapid X-ray variability is associated with the activation of new flares in the corona above the accretion disk. X-ray reverberation mapping is the technique of using observations of the iron line response, or ``echo'', to sudden changes in the continuum to study the accretion disk and black hole. In principle this may be used to determine the geometry of the X-ray emission and the black hole spin and mass. Such observations will be within the capabilities of the next generation of X-ray observatories.

• Other classes of object. Iron lines are observed in other classes of object in addition to Seyfert galaxies. In quasars the strength of the iron line decreases with increasing luminosity. This may be because the more luminous sources accreting closer to the Eddington limit and more highly ionized. The observation of iron lines in LLAGN may determine whether the accretion with low rates is an ADAF or a thin disk. Weak iron lines have also been seen in LLAGN suggesting their low accretion rate flows are thin disks as opposed to geometrically thick ADAFs. In radio loud AGN, broad iron lines and reflection humps are weak or absent, perhaps because the reflection signature is swamped by a beamed continuum. All of these require further detailed observations. In GBHC the accretion disk is ionized and the reflection spectra show smeared absorption and emission feature, and there is debate as to the precise nature of the accretion flow within a few tens of Schwarzschild radii of the black hole.

Over the past decade observations of the broad iron line have provided an unprecedented probe of the region within a few tens of Schwarzschild radii of the black hole event horizon. The next generation of X-ray observatories, beginning with XMM-Newton, will address many of the puzzling questions we have, and significantly enhance our understanding of these enigmatic objects.

Acknowledgements

We thank Mateusz Ruszkowski for comments on the manuscript. ACF thanks the Royal Society for support. CSR appreciates support from Hubble Fellowship grant HF-01113.01-98A. This grant was awarded by the Space Telescope Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS 5-26555.