1995A&A...303L..41M

We examine the effects of a nuclear jet expelled from an accretion disk on the UV-to-X-ray spectra of quasars. The base of the jet is immersed in the UV radiation field from the disk and heats up to the Compton-temperature scattering the UV photons from the disk into the soft X-ray range. Unsaturated comptonization leads to a power law extension of the UV bump spectrum to higher frequencies with spectral index α_s≳1.7 (S_ν ∝ ν^–αs_^). In the keV range, a nonthermal hard X-ray component with flatter spectral index shows up. This emission component is assumed to be associated with nonthermal processes in the jet. In radio-louds, where the jet is highly collimated and relativistic, particle acceleration occurs at a large distance from the disk, producing a boosted α_h∼0.5 hard X-ray spectrum at a low radiation compactness. In radio-quiets, the jet is slow and highly dissipative a few scale heights above the disk, producing hard X-rays with spectral index α_h∼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 spectra of several quasars, we find that soft X-ray excesses in the keV range occur when the kinetic power of the jet is of the same order as the UV luminosity.