Astronomy and Astrophysics – Astrophysics
Scientific paper
2004-10-07
Astrophys.J. 608 (2004) 80-94
Astronomy and Astrophysics
Astrophysics
Scientific paper
10.1086/392529
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 $\alpha_{\rm ox}$ 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_{\rm e} = 90 - 500 keV), or a nonthermal electron distribution in the plasma. We put an upper limit on the disk truncation radius, $r_{\rm tr} \leq 40 R_{\rm S}$. The modeled accretion rate is high, $\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.
Siemiginowska Aneta
Sobolewska Malgorzata A.
Zycki Piotr T.
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