2004ApJ...608...80S

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 α_oxparameter), the optical depth of the Comptonizing plasma has to be rather low (τ=0.02-0.25 in the corona above the disk, and τ=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≤40R_S. The modeled accretion rate is high, m{dot}>0.2M{dot}_Edd, which may suggest that high-z radio-quiet quasars are analogs of X-ray binaries in their high or very high state.