High-redshift radio-quiet quasars: Exploring the parameter space of accretion models. I. Hot semispherical flow

Two families of models are currently considered to describe an accretion flow onto black holes and production of the observed X-ray radiation: (1) a standard cold accretion disk with a hot corona above it and (2) an outer truncated accretion disk with a hot semispherical inner flow. We compute spectra in the scenario with a hot inner flow surrounded by a truncated accretion disk covered by a hot corona and test the results on a sample of high-redshift (z > 4) quasars observed with Chandra. We find that in order to reproduce the ratio of optical to X-ray fluxes (the alpha(ox) parameter), the optical depth of the Comptonizing plasma has to be rather low (tau = 0.02-0.25 in the corona above the disk, and tau = 0.10-0.70 in the hot inner flow). This, together with the observed X-ray photon indices, implies either a high temperature in a thermal plasma (kT(e) = 90-500 keV) or a nonthermal electron distribution in the plasma. We put an upper limit on the disk truncation radius, r(tr) less than or equal to 40R(S). The modeled accretion rate is high, (m) over dot > 0.2(M) over dot(Edd) , which may suggest that high-z radio-quiet quasars are analogs of X-ray binaries in their high or very high state.