The Hard VHE γ-Ray Emission in High-Redshift TeV Blazars: Comptonization of Cosmic Microwave Background Radiation in an Extended Jet?

Abstract

Observations of very-high-energy (VHE; E > 250 GeV) γ-ray emission from several blazars at z > 0.1 have placed stringent constraints on the elusive spectrum and intensity of the intergalactic infrared background radiation (IIBR). Correcting the observed VHE spectrum for γγ absorption, even by the lowest plausible level of the IIBR, provides evidence for a very hard (photon spectral index Γph < 2) intrinsic source spectrum out to TeV energies. Such a hard VHE γ-ray spectrum poses a serious challenge to the conventional synchrotron self-Compton (SSC) interpretation of the VHE emission of TeV blazars and suggests the emergence of a separate emission component beyond a few hundred GeV. Here we propose that such a very hard, slowly variable VHE emission component in TeV blazars may be produced via Compton upscattering of cosmic microwave background (CMB) photons by shock-accelerated electrons in an extended jet. For the case of 1ES 1101–232, this component could dominate the bolometric luminosity of the extended jet if the magnetic fields are of the order of typical intergalactic magnetic fields (B ∼ 10 μG) and if electrons are still being accelerated out to TeV energies (γ ≳ 4 × 106) on kiloparsec scales along the jet.