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2.2 The Optical-UV Continuum

The optical-UV feature in Fig. 2 is referred to as the ``big blue bump''. which in many sources contains a significant fraction of the bolometric luminosity. One possible origin is thermal emission from an accretion disk, 25 - 27 and spectral fits using disk spectra have the advantage of being able to estimate both the luminosity (accretion rate) and the central mass. However, a part of the UV bump may arise from reprocessing of radiation from the central hard X-ray source, which seems indicated by the similar variability patterns seen in UV, optical and X-rays. 28 This behaviour has only been observed in a few objects (such as NGC 5548), and it is unclear if the same applies to other sources with a lower X-ray luminosity. 29

An alternative model, which involves dense, compact clouds with a high opacity, has been proposed by Guilbert and Rees 30 and independently by Lightman and White. 31 Located close to the central hard X-ray source, such clouds would emit thermal radiation, similar to the big bump. Collin-Souffrin 32 pointed out that since the X-ray and UV continua then would arise in the same physical region, the corresponding variability timescales should be approximately the same. This is not the case for at least high-luminosity AGN, where the X-ray fluctuations are more rapid. Moreover, the small total emitting surface of the clouds would imply a negligible UV luminosity in comparison with the bolometric one, which also is not observed.