Hard MeV–GeV spectra of blazars

Toruń Centre for Astronomy, Nicolaus Copernicus University, ul. Gagarina 11, 87100 Toruń, Poland
e-mail: kat@astro.uni.torun.pl

Received: 7 March 2011
Accepted: 11 October 2011

Abstract

Aims. Very high energy (VHE) gamma-ray emission from a distant source (z ≳ 0.2) can be efficiently absorbed by means of the electron-positron pair creation process. Analyses of the unabsorbed spectra imply that the intrinsic TeV emission of some blazars is hard, with spectral indices of 0.5 < α < 1. The absorption depends on the level of extragalactic background light (EBL) that is difficult to measure directly. This implies that it is difficult to estimate the slope of the intrinsic TeV emission. To test our blazar emission scenario that is capable of reproducing the hard spectra, we therefore used the observations made by the Fermi Gamma-ray Space Telescope in the unabsorbed MeV−GeV energy range.

Methods. We assume that the X-ray and gamma-ray emission of TeV blazars is produced in a compact region of a jet uniformly filled by particles of relatively high energy (γ ≳ 10³, E = γm_ec²). In other words, we assume a low energy cut-off in the particle energy distribution. The emission produced by the particles with this energy spectrum can explain hard intrinsic spectra in the energy range from MeV up to TeV. We demonstrate how to estimate the basic physical parameters of a source in this case and how to explain the observed spectra by a precise simulation of the particle energy evolution.

Results. To test our estimation methods, we use the observations of two blazars with exceptionally hard spectral indices (α ≲ 0.5) in the MeV − GeV range and known redshifts: RGB J0710+591 and 1ES 0502+675. The estimated values of the Doppler factor and magnetic field are compared with our numerical simulations, which confirm that the particle energy distribution with a low energy cut-off can explain the observed hard spectra well. In addition, we demonstrate that the radiative cooling caused by the inverse-Compton emission in the Klein-Nishina regime may help us to explain the hard spectra.