2.3.1 The Soft Excess

More than half of all Seyferts observed by EXOSAT showed an excess above the presumed underlying power law, in the range 0.1-2 keV. In fact, there is growing evidence of soft X-ray excesses being present in all AGN. ³⁴ The excess has been connected with a high-energy tail of the UV bump, ³⁵ but this suggestion awaits observational verification. Using the Shakura-Sunyaev model to fit the excess may be inappropriate, since a relevant temperature often requires 1. Another complication is that the disk should have a high inclination, ³⁶ which implies an optical emission much fainter than for a face-on disk. However, sources with detected excesses show an optical emission typical of other AGN. ³⁴

The alternative origin due to Comptonization in a corona ³⁷ may have been ruled out by the short variability timescale of the excess. ³⁸ However, the mechanisms behind the excess and the hard X-rays may operate in approximately the same region, since their variability timescales are roughly similar, as observed ^{39, 40} in NGC 5548 and MCG-6-30-15. Possible mechanisms include Comptonization in an optically thin part of the accretion disk (next section) or a thermalized pair plasma. ⁴¹