The spectrum of the Broad Line Region and the high-energy emission of powerful blazars

Abstract

High-energy emission (from the X-ray through the gamma-ray band) of Flat Spectrum Radio Quasars is widely associated with the inverse Compton (IC) scattering of ambient photons, produced either by the accretion disk or by the Broad Line Region, by high-energy electrons in a relativistic jet. In the modelling of the IC spectrum one usually adopts a simple black-body approximation for the external radiation field, though the real shape is probably more complex. The knowledge of the detailed spectrum of the external radiation field would allow to better characterize the soft-medium X-ray IC spectrum, which is crucial to address several issues related to the study of these sources. Here we present a first step in this direction, calculating the IC spectra expected by considering a realistic spectrum for the external radiation energy density produced by the BLR, as calculated with the photoionization code CLOUDY. We find that, under a wide range of the physical parameters characterizing the BLR clouds, the IC spectrum calculated with the black-body approximation reproduces quite well the exact spectrum for energies above few keV. In the soft energy band, instead, the IC emission calculated using the BLR emission shows a complex shape, with a moderate excess with respect to the approximate spectrum, which becomes more important for decreasing values of the peak frequency of the photoionizing continuum. We also show that the high-energy spectrum shows a marked steepening, due to the energy dependence of the scattering cross section, above a characteristic energy of 10-20 GeV, quasi independent on the Lorentz factor of the jet.