Compton reflection and iron fluorescence in AGN and GBHCs

Any cold, optically-thick matter in the vicinity of an accreting black hole, such as the accretion disk, can intercept and reprocess some fraction of the hard X-ray continuum emission, thereby imprinting atomic features into the observed spectrum. This process of ‘X-ray reflection’ primarily gives rise to a broad reflection ‘hump’ peaking at ∼ 30 keV and an iron emission line at 6.4 keV. In this review, I briefly describe the physics of this process before reviewing the observations of these features in active galactic nuclei (AGN) and Galactic black hole candidates (GBHCs). In some AGN, Seyfert galaxies in particular, the iron line is found to be very broad and asymmetric. It is believed that such lines arise from the innermost regions of the accretion disk, with mildly-relativistic Doppler shifts and gravitational redshifts combining to produce the line profile. Hence, such lines give us a direct observational probe of the region within several gravitational radii of the black hole. The complications that plague similar studies of GBHCs, such as disk ionization and the possibly of inner disk disruption, are also addressed. I conclude with a discussion of iron line reverberation, i.e. temporal changes of the iron line as ‘echos’ of large X-ray flares sweep across the accretion disk. It is shown that interesting reverberation effects, such as a definitive signature of extremal Kerr geometry, is within reach of high throughput spectrometers such as Constellation-X. 1. X-ray reflection features The environment of an accreting black hole can contain optically-thick, cold matter in addition to the more exotic high-energy plasma that gives rise to the hard X-ray continuum. For example, at least some black hole accretion disks are thought to be radiatively-efficient (in the sense that they radiate locally almost all of the energy that is deposited locally by viscous processes) and hence cold. The proximity of this cold matter to the hot X-ray emitting plasma has an important consequence — the cold matter intercepts and reprocesses some fraction of the primary (featureless) X-ray continuum and hence imprints atomic features in the observed spectrum. These, the so called ‘Compton reflection and