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1. INTRODUCTION

In recent years, observations with both ground-based and space-based instruments have led to realization that most, if not all, nucleated galaxies harbor a massive black hole at their center (Kormendy & Richstone 1995; Magorrian et al. 1998). While many of these black holes appear to be relatively isolated, some fraction accrete significant amounts of material from the surrounding galaxy. The angular momentum of the incoming material leads to the formation of a flattened rotating disk - the accretion disk. The gravitational potential energy of material flowing through the accretion disk is converted into radiative (i.e. electromagnetic) and kinetic energy. These powerful and compact energy sources, observed in approximately 1-10% of galaxies are termed active galactic nuclei (AGN). AGN are also observed to be copious X-ray emitters. These X-rays are thought to originate from the innermost regions of an accretion disk around a central supermassive black hole. Since the accretion disk itself is expected to be an optical/UV emitter, the most likely mechanism producing the X-rays is inverse Compton scattering of these soft photons in a hot and tenuous corona that sandwiches the accretion disk. Thus, in principle, the study of these X-rays should allow the immediate environment of the accreting black hole as well as the exotic physics, including strong-field general relativity, that operates in this environment to be probed.

This review discusses how, in the past decade, X-ray astronomy has begun to fulfill that promise. Guided by observations with the Ginga, ASCA, RXTE and BeppoSAX satellites, there is a broad concensus that X-ray irradiation of the surface layers of the accretion disk in a class of AGN known as Seyfert 1 galaxies gives rise to fluorescent Kalpha emission line of cold iron via the process of ``X-ray reflection''. Since this line is intrinsically narrow in frequency, the observed energy profile of the line is shaped by both special relativistic (i.e. Doppler shifting) and general relativistic (i.e. gravitational redshifting and light bending) effects into a characteristic skewed profile predicted over a decade ago (Fabian et al 1989) and first clearly seen in ASCA data (Tanaka et al 1995). Since these lines are typically broadened to a full-width half maximum of 5 x 104 km s-1 or more, they are often referred to as ``broad iron lines''. After discussing the physical processes responsible for the production of these spectral signatures, we will summarize the current observational status of broad iron line studies. We will show how current observations are already addressing the nature of the accretion disk within a few gravitational radii of the black hole. Observations of the broad iron line also provide valuable insights into the physical differences behind AGN of differing luminosities and type. Finally, we discuss and attempt to predict the results that will emerge from high throughput X-ray spectroscopy with XMM-Newton, Constellation-X and XEUS. We argue that these future data will provide unprecedented constraints on the spacetime geometry near the black hole (thereby measuring the spin of the black hole), well as the physical nature of the accretion disk.

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