X-ray emission has been found to be a universal characteristic of all AGN and quasars. The X-ray luminosity of these objects is in the range ≈ 1042-1048 erg sec-1 and accounts for a considerable fraction of their bolometric luminosity. The X-ray flux shows large amplitude variability on a time scale of days, hours and in some cases hundreds of seconds. This implies that the X-ray emission has its origin very close to the central objects. For energies ≿ 1 keV the X-ray spectra appear to have a simple power law shape, which is modified at lower energies owing to absorption at the source. Improved energy resolution has shown the presence of emission and absorption features, which prove to be important diagnostics of conditions close to the active nucleus. At energies below a few keV, AGN and quasars contribute a significant fraction of the observed X-ray background (XRB). The observed spectra of AGN and quasars in the 2–10 keV range appear to be different from the spectrum of the X-ray background and a way has to be found to reconcile AGN spectra with the XRB, if the AGN indeed contribute a substantial fraction to it at high energy.
In the following sections we will discuss some important X-ray missions, elementary steps used in the analysis of data, X-ray surveys, luminosity functions and broad band properties of AGN and quasars. We will consider X-ray spectra in Chapter 11.
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