3.4.1. The soft excess
Low energy X-ray data on many AGN cannot be fitted by extrapolation of the power law that is used to describe the medium energy component. The flux observed at low energies ( 0.5keV) is systematically larger than that predicted by this extrapolation. Since the presence of cold matter along the line of sight decreases the low energy X-ray flux we can deduce that there are no large amounts of cold material in excess of that within our Galaxy. The soft excess (common also in Seyfert galaxies) is poorly characterized as it extends into the low energy region in which the flux is completely absorbed by interstellar matter, because only few energy resolution elements are available (this is expected to change with the launch of the XMM satellite in 1999) and because the amount of absorbing matter on the line of sight is not known. One way of fitting the data is by an optically thin thermal emission model of k . T 0.2 keV, another is to use a power law of photon index -2.7 [Leach McHardy & Papadakis 1995]. These fits should be taken as a mathematical description of the shape of the emission rather than as a true physical description of the emission.
Simultaneous observations in the UV and X-ray domains in a set of objects including 3C 273 [Walter et al. 1994] and [Walter & Fink 1993] indicate that the parameters of the soft excess and those of the blue bump are correlated. Should this be confirmed, it would show that the soft excess is the high energy tail of the blue bump. This high energy end of the blue bump would then occur at roughly the same energy for nuclei of very different luminosities, a fact unexpected in standard accretion disc models in which the maximum temperature is expected to decrease with the 1/4 power of the luminosity [Courvoisier & Clavel 1991].