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 - 0.15 in the corona above the disk , and \tau = 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 ( 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 } \leq 40 R _ { S } .
The modeled accretion rate is high , \dot { m } > 0.2 \dot { M } _ { Edd } , which may suggest that high- z radio quiet quasars are analogs of X-ray binaries in their high or very high state .