The extended jet of AP Librae: Origin of the very high-energyγ-ray emission?

Abstract

The low-frequency peaked BL Lac object (LBL) AP Librae exhibits very-high-energy (VHE, $E>100$GeV) $\gamma$-ray emission and hosts an extended jet, which has been detected in radio and X-rays. The jet X-ray spectral index implies an inverse Compton origin. These observations are unusual for LBLs calling for a consistent explanation of this extraordinary source. The observationally constrained parameters necessary to describe the core emission within the standard one-zone model are unable to explain the broad-band spectrum, even if observationally unconstrained external photon fields are taken into account. We demonstrate that the addition of the extended jet emission successfully reproduces the total spectral energy distribution. In particular, the VHE radiation is produced in the $>100\,$kpc long extended jet via inverse Compton scattering of cosmic microwave background photons by highly relativistic electrons. We present several ways to test this theory. The extended jet is weakly magnetized ($B_0 = 2.5\,\mu$G), while its minimum and maximum electron Lorentz factors are $\gamma_{min}=60$ and $\gamma_{max}=5\times 10^6$, respectively. The electron spectral index is $s=2.6$. These parameters are comparable to parameters of other blazars with extended X-ray jets dominated by inverse Compton scattering.