On the Nature of MeV Blazars

Abstract

Broadband spectra of the flat-spectrum radio quasars (FSRQs) detected in the high-energy γ-ray band imply that there may be two types of such objects: those with steep γ-ray spectra, hereafter called MeV blazars, and those with flat γ-ray spectra, GeV blazars. We demonstrate that this difference can be explained in the context of the external radiation Compton (ERC) model using the same electron injection function. A satisfactory unification is reachable, provided that (1) spectra of GeV blazars are produced by internal shocks formed at the distances where cooling of relativistic electrons in a jet is dominated by Comptonization of broad emission lines, whereas spectra of MeV blazars are produced at the distances where cooling of relativistic electrons is dominated by Comptonization of near-IR radiation from hot dust, and (2) electrons are accelerated via a two-step process and their injection function takes the form of a double power law, with the break corresponding to the threshold energy for the diffusive shock acceleration. Direct predictions of our model are that, on average, variability timescales of the MeV blazars should be longer than variability timescales of the GeV blazars, and that both types of the blazar phenomenon can appear in the same object.