|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
The continuum of radio-quietAGN varies on all observable time scales, with amplitude up to a factor of 50 or so. The variability character depends on the wavelength, and there are correlations among the variations in different energy bands, as well as astrophysically important upper limits to the time delays between these variations, as detailed below.
The electromagnetic spectrum of radio-quiet AGN, after subtraction of the stellar continuum, extends from ~ 1 mm to ~ 100 keV with a prominent broad peak in the UV-extreme UV (EUV) range (for f versus ; Sanders et al 1989). The spectrum from 1200-5000 Å strongly suggests that the broad peak is primarily thermal emission from a very dense medium, probably an accretion disk (Lynden-Bell 1969, Shields 1978, Malkan & Sargent 1982; but see Ferland et al 1990). The temperature of the disk, however, is not set primarily by viscous effects, as initially thought, but by irradiation from the central X-ray source, as demonstrated by the recent variability studies (see below).The spectrum is not known in the range 1000 Å to ~ 0.1 keV.