Where within the 3C 84 jet are $\gamma$-rays produced?

TL;DR

This study uses VLBI polarisation data of 3C 84 to localize gamma-ray emission regions within its jet, suggesting gamma-rays originate near the flare site through synchrotron self-Compton processes.

Contribution

It provides observational evidence linking polarisation increases at parsec scales to gamma-ray flares, advancing understanding of gamma-ray production sites in AGN jets.

Findings

Polarisation increases near the gamma-ray flare site.

Gamma-ray emission likely occurs in regions with enhanced polarisation.

Supports synchrotron self-Compton as the gamma-ray production mechanism.

Abstract

The location of $\gamma$-ray creation and emission within extra-galactic jets is a matter of active debate. One particularly well-suited source to pinpoint the location is the nearby, bright radio galaxy 3C 84, harbouring a powerful jet. Here we investigate the origin of $\gamma$-rays measured during a recent $\gamma$-ray flare, by analysing the linear polarisation signal of close-in-time very long baseline interferometry (VLBI) observations at centimetre and millimetre wavelengths. While 3C 84 is overall almost unpolarised, we find that close-in-time to the $\gamma$-ray flare peak regions at parsec-scale distances from the central engine shows a fractional linear polarisation increase. Under the physically well-motivated assumption of a causal relation between this polarisation enhancement and the $\gamma$-ray flare, and combined with insights from concurrent X-ray polarisation measurements, the $\gamma$-rays being created in this region is a physically motivated scenario, in a process consistent with synchrotron self-Compton.