Cloud model of the mean quasar spectrum

We assume a distribution of clouds optically thick for electron scattering (OTCM) which are moderately optically thin for absorption and we consider them as a model of the mean quasar spectrum of Laor et al. (1997). We show that the model is particularly sensitive to the value of the ionization parameter ξ and that for ξ ∼ 500 the model well reproduces the optical/UV/X-ray mean quasar spectrum, in agreement with the estimates of the ionization parameter based on the energy of the iron Kα line. We cannot definitively reject synchrotron emission as a source of primary radiation but we favor the model in which the hard X-ray emission is produced by Compton scattering of soft photons in a central hot medium surrounded by cool clouds. In such a model clouds are located typically at the distance of ∼ 12RSchw, with the covering factor about 0.88 and the radius of hot plasma is ∼ 9RSchw. The model explains optical/UV emission as predominantly due to the dark sides of the clouds and the soft X-ray emission as due to the reflection by the irradiated sides of the clouds. Therefore, atomic features are expected in these bands although they are hardly present in the observational data. The kinematical effects connected with the cloud motion affect those features but do not remove them. The level of the primary emission required to model the mean quasar spectrum is too low to reproduce the equivalent width of the iron Kα line correctly but more detailed computations may resolve this problem.