Where within the 3C 84 jet are ${\hat{I}}^3$-rays produced?

Abstract

The location of ${\hat{I}}^3$-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 ${\hat{I}}^3$-rays measured during a recent ${\hat{I}}^3$-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 ${\hat{I}}^3$-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 ${\hat{I}}^3$-ray flare, and combined with insights from concurrent X-ray polarisation measurements, the ${\hat{I}}^3$-rays being created in this region is a physically motivated scenario, in a process consistent with synchrotron self-Compton.

10 pages, 4 figures, 2 appendices, accepted for publication by A&A