The origin of soft X-rays in luminous AGN

Abstract

We predict that the UV bump spectrum of quasars extends into the soft X-ray range with a steep power law. For radio-louds, the spectral index is alpha_s ~ 1.7 (S_nu = S_o nu^-alpha), and for radio-quiets it should be somewhat steeper alpha_s >~ 1.7. The prediction is based on a simple scenario in which all quasars, radio-quiets and radio-louds, are assumed to have jets close to their accretion disks. Soft X-rays are produced via unsaturated Comptonization of UV photons from the disk in the (marginally) optically thick jet base. Nonthermal hard X-rays are produced further downstream. The scenario requires that (i) the jet kinetic luminosity is always of the same order as the UV luminosity and (ii) that the nonthermal radiative efficiency is ~ 10%. In radio-quiets, the jets should have speeds of ~ 0.3c and a low Mach number leading to rapid decollimation and turbulent mass entrainment within the central parsec.