Shaping the GeV-spectra of bright blazars

Abstract

The non-thermal spectra of jetted active galactic nuclei show a variety of shapes in their low- and high energy components. In some of the brightest Fermi-LAT blazars, prominent spectral breaks at a few GeV have been regularly detected, which is inconsistent with conventional cooling effects. We study the effects of continuous time-dependent injection of electrons into the jet with differing rates, durations, locations, and power-law spectral indices, and evaluate its impact on the ambient emitting particle spectrum at a given snapshot time in the framework of a leptonic blazar emission model. The emitting electron spectrum is calculated by Compton cooling the continuously injected electrons, where target photons are assumed to be provided by the accretion disk and broad line region. We calculate the non-thermal photon spectra produced by inverse Compton scattering of these external target radiation fields using the full Compton cross-section in the head-on approximation. By means of a comprehensive parameter study we present the resulting ambient electron and photon spectra, and discuss the influence of each injection parameter. Varying the injection parameters has a notable influence on the spectral shapes, which can be used to set constraints on the injection scenarios. We apply our model to the flare state spectral energy distribution of 3C454.3, and to the FSRQ PKS1510-089. For both sources we show two different model fits, corresponding to different injection scenarios. In all four injection scenarios impulsive particle injection is disfavored. Our model aims towards bridging jet emission with acceleration models using a phenomenological approach. Blazar spectral data can be analyzed with this model to constrain injection parameters, in addition to the conventional parameter values of steady-state emission models, if sufficient broad multifrequency coverage is provided.