Fluorescent iron lines as a probe of astrophysical black hole systems

With most physicists and astrophysicists in agreement that black holes do indeed exist, the focus of astrophysical black hole research has shifted to the detailed properties of these systems. Nature has provided us with an extremely useful probe of the region very close to an accreting black hole — X-ray irradiation of relatively cold material in the vicinity of the black hole can imprint characteristic features into the X-ray spectra of black hole systems, most notably the Kα fluorescent line of iron. Detailed X-ray spectroscopy of these features can be used to study Doppler and gravitational redshifts, thereby providing key information on the location and kinematics of the cold material. This is a powerful tool that allows us to probe within a few gravitational radii, or less, of the event horizon. Here, we present a comprehensive review of relativistic iron line studies for both accreting stellar mass black holes (i.e., Galactic Black Hole Candidate systems; GBHCs), and accreting supermassive black holes (i.e., active galactic nuclei; AGN). We begin with a pedagogical introduction to astrophysical black holes, GBHCs, AGN, and accretion disks (including a brief discussion of recent work on the magnetohydrodynamical properties of accretion disks). We then discuss studies of relativistic iron lines in the AGN context, and show how differences between classes of AGN can be diagnosed using X-ray spectroscopy. Furthermore, through a detailed discussion of one particular object (MCG–6-30-15), we illustrate how the exotic physics of black hole spin, such as the Penrose and Blandford-Znajek processes, are now open to observational study. We proceed to discuss GBHCs, which turn out to possess rather more complicated X-ray spectra, making robust conclusions more difficult to draw. However, even in these cases, modern X-ray observatories are now providing convincing evidence for relativistic effects. We conclude by discussing the science that can be addressed by future X-ray observatories.