# Constraints on Proton Synchrotron Origin of Very High Energy Gamma Rays from the Extended Jet of AP Librae

## TL;DR

This paper examines whether proton synchrotron radiation can explain the very high energy gamma rays from AP Librae's extended jet, challenging previous inverse Compton models and highlighting the extreme conditions required.

## Contribution

It introduces a proton synchrotron model for high-energy emission from extended jets, contrasting with prior inverse Compton models, and discusses the extreme physical conditions needed.

## Key findings

- Proton synchrotron model requires very high proton energies and magnetic fields (~1 mG).
- The model demands jet powers exceeding the Eddington luminosity of AP Librae.
- X-ray emission from other jets could also be explained by proton synchrotron under similar conditions.

## Abstract

The multi-wavelength photon spectrum from the BL Lac object AP Librae extends from radio to TeV gamma rays. The X-ray to very high energy gamma ray emission from the extended jet of this source has been modeled earlier with inverse Compton (IC) scattering of relativistic electrons off the CMB photons. The IC/CMB model requires the kpc scale extended jet to be highly collimated with bulk Lorentz factor close to 10. Here we discuss the possibility of proton synchrotron origin of X-rays and gamma-rays from the extended jet with bulk Lorentz factor 3. This scenario requires extreme energy of protons and high magnetic field 1 mG of the extended jet with very high jet power in particles and magnetic field (which is more than 100 times the Eddington luminosity of AP Librae) to explain the very high energy gamma ray emission. Moreover, we have shown that X-ray emission from the extended jets of 3C 273 and PKS 0637-752 could be possible by proton synchrotron emission with jet powers comparable to their Eddington luminosities.