THE ORIGIN OF SOFT X-RAYS IN LUMINOUS AGN

We examine the effects of a nuclear jet expelled from an accretion dis k on the UV-to-X-ray spectra of quasars. The base of the jet is immers ed in the UV radiation held from the disk and heats up to the Compton- temperature scattering the UV photons from the disk into the soft X-ra y range. Unsaturated comptonization leads to a power law extension of the UV bump spectrum to higher frequencies with spectral index alpha(s ) greater than or similar to 1.7 (S(n)u proportional to nu(-alpha epsi lon)). In the keV range, a nonthermal hard X-ray component with flatte r spectral index shows up. This emission component is assumed to be as sociated with nonthermal processes in the jet. In radio-louds, where t he jet is highly collimated and relativistic, particle acceleration oc curs at a large distance from the disk, producing a boosted alpha(h) s imilar to 0.5 hard X-ray spectrum at a low radiation compactness. In r adio-quiets, the jet is slow and highly dissipative a few scale height s above the disk, producing hard X-rays with spectral index alpha(h) s imilar to 1 by pair cascades at a moderate radiation compactness. The jet provides the energetical link between the disk and the hard X-ray source. Modeling the broad-band spec of several quasars, we find that soft X-ray excesses in the keV range occur when the kinetic power of t he jet is of the same order as the UV luminosity.