In the previous chapter we considered how the broad band X-ray emission from quasars and AGN is related to the emission from other bands. The results from the large database on broad band fluxes are important in providing some pointers to the processes by which the X-ray emission is produced, but a detailed understanding requires observation of the spectral shape of the continuum, and any emission lines that may be present. Owing to the small collecting area of the X-ray telescopes that have so far been used, and the high energy per photon, the total number of photons received from a source is rather limited. It is therefore possible to obtain reliable spectra only for nearby AGN, which in spite of their moderate luminosity have a relatively large flux, and for the most luminous quasars. In spite of this limitation, spectral shapes have been sufficiently well established for a detailed comparison to be made with the predictions of the models described in Chapter 4. In this chapter we will consider the observation and modelling of the spectra from discrete sources as well as the diffuse background radiation in the X-ray and γ-ray domains.

X-ray spectra

The X-ray continuum for quasars and AGN in the 2–10 keV energy range can usually be well approximated by a power law. It is found that the spectral index is distributed in a narrow range in the case of AGN, but shows a somewhat wider distribution in the case of quasars.