The Nature and Origin of X-Ray Emission in Active Galaxies

Summary

Recent observations of X-ray spectra of AGNs can be used to compare the “classical” black-hole type model for such objects with the starburst scenario emphasized in this meeting. The most important aspects of the observations are the commonly observed soft X-ray absorption, the K_α line profile and intensity, the X-ray variability and the hard X-ray cut-off. Modelling the observed X-ray spectrum requires an understanding of the absorption, emission and scattering properties of neutral and ionized gases. Examples from new calculations, including all these components, are shown and compared with the observations. Progress made on the observational and theoretical sides seem to give at least some satisfactory answers to previously open questions of the black-hole model. It remains to be seen whether the starburst model can come up with an equally good explanation.

Introduction

The purpose of this review is to summarize some well known facts, as well as new X-ray observations of various types of AGNs, and to confront them with suggested models. Since X-ray properties involve emission, absorption and scattering it is useful to stop, at each sub-section, and ask the question, What are the physical properties of the emitting, absorbing and scattering material? These are related to the location of the various components, their motion and other properties. They can provide an answer to the central question posed in this meeting, about the relationship between “classical” AGNs, hosting a giant black-hole, and violent star-forming events in nuclei of galaxies.