Shaping the GeV-spectra of bright blazars

¹ Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
e-mail: lars.hunger@brainlinks-braintools.uni-freiburg.de
² Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
³ Brainlinks-Braintools, University of Freiburg, 79104 Freiburg im Breißgau, Germany

Received: 3 August 2014
Accepted: 15 February 2016

Abstract

Aims. The non-thermal spectra of jetted active galactic nuclei (AGN) show a variety of shapes and degrees of curvature in their low- and high energy components. From 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 that is observed at a given snapshot time in the framework of a leptonic blazar emission model. With this study, we provide a basis for analyzing ambient electron spectra in terms of injection requirements, with implications for particle acceleration modes.

Methods. 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 (BLR). From this setup, 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.

Results. By means of a comprehensive parameter study we present the resulting ambient electron and photon spectra, and discuss the influence of each injection parameter individually. We found that varying the injection parameters has a notable influence on the spectral shapes, which in turn can be used to set interesting constraints on the particle injection scenarios. By applying our model to the flare state spectral energy distribution (SED) of 3C 454.3, we confirm a previous suggestion that explained the observed spectral changes at a few GeV by a combination of the Compton-scattered disk and BLR radiation. We determine the required injection parameters for this scenario. We also show that this spectral turn-over can also be understood as Compton-scattered BLR radiation only, and provide the corresponding injection parameters. Here the spectral turn-over is explained by a corresponding break in the ambient electron spectrum. In a similar way, we also applied our model to the FSRQ PKS 1510-089, and present two possible model fits. Here, the GeV-spectrum is either dominated by Compton-scattered accretion disk radiation or is a combination of Compton-scattered disk and BLR radiation. We provide the required injection parameters for these fits. In all four scenarios, we found that impulsive particle injection is disfavored.

Conclusions. The presented injection model that is embedded in a leptonic blazar emission model for external Compton-loss dominated jets of AGN 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.