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

TL;DR

This paper explains the very-high-energy gamma-ray emission of AP Librae by modeling its extended jet's inverse Compton scattering of cosmic microwave background photons, revealing the jet's role in broad-band emission.

Contribution

It demonstrates that including the extended jet emission in models explains the broad-band spectrum of AP Librae, a feature not accounted for by standard one-zone models.

Findings

VHE gamma-ray emission is produced in the extended jet via inverse Compton scattering.

The extended jet is weakly magnetized with specific electron Lorentz factors and spectral index.

The model parameters are consistent with other blazars with inverse Compton dominated X-ray jets.

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.