High redshift radio quiet quasars - exploring the parameter space of accretion models. Part I: hot semi-spherical flow

Abstract

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 ${\alpha }_{\mathrm{ox}}$ parameter), the optical depth of the Comptonizing plasma has to be rather low ($\tau =0.02-0.15inthecoronaabovethedisk,and$τ= 0.15 - 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 ($k{T}_{\mathrm{e}}=90-500keV),oranonthermalelectrondistributionintheplasma.Weputanupperlimitonthedisktruncationradius,$r_{\rm tr} \leq 40 R_{\rm S}$.Themodeledaccretionrateishigh,$\dot{m} > 0.2 Eddington rate, which may suggest that high-z radio quiet quasars are analogs of X-ray binaries in their high or very high state.