The Extended Jet In AP Librae As The Source Of The VHE $γ$-ray Emission

Abstract

Most modeling attempts of blazars use a small emission zone located close to the central black hole in order to explain the broad-band spectral energy distribution. Here we present a case where additionally to the small region a $>$kpc-scale jet is required to successfully reproduce the spectrum and especially the TeV emission, namely the low-frequeny peaked BL Lac object AP Librae detected in the TeV domain by the H.E.S.S. experiment. Given that other parts of the spectral energy distribution follow the characteristics implied by the source classification, the inverse Compton component spans 10 orders of magnitude, which cannot be reproduced by the one-zone model. Additionally, observational constraints in both the synchrotron and inverse Compton compoenent strongly constrain the parameters of a self-consistent model ruling out the possibility of TeV photon production in the vicinity of the galactic center. We discuss the possibility that the TeV radiation is emitted by highly energetic particles in the extended, arcsec-scale jet, which has been detected at radio and X-ray energies. The slope of the jet X-ray spectrum indicates an inverse Compton origin, and an extrapolation to higher energies coincides with a break feature in the $γ$-ray band. Modeling the jet emission with inverse Compton scattering of the cosmic microwave background results in an excellent fit of the radio, X-ray and TeV emission. Implications will be discussed, such as properties of the jet, acceleration scenarios, and observations to test the model. If confirmed, large scale jets are able to efficiently accelerate particles and to keep relativistic speeds up to distances of several 100kpc.