MATTER UNDER STRONG GRAVITY: THE NUCLEAR REGIONS OF AGN

Abstract

X-ray observations of Seyfert galaxies and quasars currently provide the most powerful way to study the environments of supermassive black holes. Our ability to probe these environments, some of the most extreme found in nature, is rapidly improving driven by dramatic advances in X-ray spectroscopic instrumentation. In addition to the well known 'hard X-ray power law' that is probably formed in a hot 'corona' above a cooler accretion disc, substantial spectral complexity has also been revealed. For example, soft X-ray excess emission above the power law is often seen at low X-ray energies, and this emission is thought to be associated with the inner disc. Atomic absorption/emission processes in nuclear gas flows are also observed to imprint characteristic features on X-ray spectra. Iron Kα emission lines from 6.40–6.97 keV are formed at least partially by fluorescence in the inner disc, and absorption edges seen mainly below 2 keV reveal major flows of ionized gas in the nucleus. Studies of the complex X-ray spectra of Seyferts and quasars have undergone a revolution wit the launches of Chandra in 1998 and XMM-Newton in 1999. The era of high-resolution X-ray spectroscopy has been initiated by the Chandra and XMM-Newton grating spectrometers. These detectors provide X-ray spectra with 6–100 times higher spectral resolution than those previously available, and they have dramatically enlarged the arsenal of diagnostics available for investigating Seyfert/quasar nuclei. The new generation of X-ray observatories is used to intensively address some of the leading questions about Seyfert/quasar X-ray emission. Some of the most important scientific issues for the next years are: — Matter under strong gravity: Why are relativistically blurred Fe K lines so rare? — What are the extreme X-ray spectral and variability properties of ultrasoft Narrow-Line Seyfert 1 galaxies telling us about Seyferts and quasars more generally? — What is the physical nature of the sharp spectral cut-off in Seyfert galaxies? While the advances expected from the new generation of X-ray observatories are so large and widespread that precise predictions are difficult, it is believed that particularly important progress will be made towards answering these central questions. Below is explained why the questions I have stated are fundamental and timely.