The very high energy (VHE) γ-ray spectra of some TeV Blazars, after being corrected for absorption in the extragalactic background light (EBL), appear unusually hard, which poses challenges to conventional acceleration and emission models. We investigate the parameter space that allows the production of such hard TeV spectra within time-dependent leptonic models, both for synchrotron self-Compton (SSC) and external Compton (EC) scenarios. In the context of interpretation of very hard γ-ray spectra, time-dependent considerations become crucial because even extremely hard, initial electron distributions can be significantly deformed due to radiative energy losses. We show that very steep VHE spectra can be avoided if adiabatic losses are taken into account. Another way to keep extremely hard electron distributions in the presence of radiative losses, is to assume stochastic acceleration models that naturally lead to steady-state relativistic, Maxwellian-type particle distributions. We demonstrate that in either case leptonic models can reproduce TeV spectra as hard as E γ dN/dE γ ∝ E γ . Unfortunately this limits, to a large extend, the potential of extracting EBL from γ-ray observations of blazars.

Aims. We investigate the role of the second synchrotron self-Compton (SSC) photon generation to the multiwavelength emission from the compact regions of sources that are characterized as misaligned blazars. For this, we focus on the nearest high-energy emitting radio galaxy Centaurus A and we revisit the one-zone SSC model for its core emission. Methods. We have calculated analytically the peak luminosities of the first and second SSC components by first deriving the steadystate electron distribution in the presence of synchrotron and SSC cooling, and then by using appropriate expressions for the positions of the spectral peaks. We have also tested our analytical results against those derived from a numerical code where the full emissivities and cross-sections were used. Results. We show that the one-zone SSC model cannot account for the core emission of Centaurus A above a few GeV, where the peak of the second SSC component appears. We thus propose an alternative explanation for the origin of the high-energy ( 0.4 GeV) and TeV emission, where these are attributed to the radiation emitted by a relativistic proton component through photohadronic interactions with the photons produced by the primary leptonic component. We show that the required proton luminosities are not extremely high, i.e. ∼10 43 erg/s, provided that the injection spectra are modelled by a power law with a high value of the lower energy cutoff. Finally, we find that the contribution of the core emitting region of Cen A to the observed neutrino and ultra-high-energy cosmic-ray fluxes is negligible.

The spectral shape of radiation due to Inverse Compton Scattering is analyzed, in the Thomson and the Klein-Nishina regime, for electron distributions with exponential cut-off. We derive analytical, asymptotic expressions for the spectrum close to the maximum cut-off region. We consider monoenergetic, Planckian and Synchrotron photons as target photon fields. These approximations provide a direct link between the distribution of parent electrons and the up-scattered spectrum at the cut-off region.

Evidence for very hard, intrinsic γ-ray source spectra, as inferred after correction for absorption in the extragalactic background light (EBL), has interesting implications for the acceleration and radiation mechanisms acting in blazars. A key issue so far has been the dependance of the hardness of the γ-ray spectrum on different existing EBL models. The recent Fermi observations of Mkn 501 now provide additional evidence for the presence of hard intrinsic γ-ray spectra independent of EBL uncertainties. Relativistic Maxwellian-type electron energy distributions that are formed in stochastic acceleration scenarios offer a plausible interpretation for such hard source spectra. Here we show that the combined emission from different components with Maxwellian-type distributions could in principle also account for more softer and broader, power law-like emission spectra. We introduce a "leading blob" scenario, applicable to active flaring episodes, when one (or few) of these components become distinct over the "background" emission, producing hard spectral features and/or hardening of the observed spectra. We show that this model can explain the peculiar high-energy characteristics of Mkn 501 in 2009, with evidence for flaring activity and strong spectral hardening at the highest γ-ray energies.

The beaming pattern of radiation emitted by a relativistically moving source like jets in microquasars, AGN and GRBs, is a key issue for understanding of acceleration and radiation processes in these objects. In this paper we introduce a formalism based on a solution of the photon transfer equation to study the beaming patterns for emission produced by electrons accelerated in the jet and upscattering photons of low-energy radiation fields of external origin (the so-called External Compton scenario). The formalism allows us to treat non-stationary, non-homogeneous and anisotropic distributions of electrons, but assuming homogeneous/isotropic and non-variable target photon fields. We demonstrate the non-negligible impact of the anisotropy in the electron distribution on angular and spectral characteristics of the EC radiation.