Where within the 3C 84 jet are γ-rays produced?

The location in which γ-ray are created and emitted within extra-galactic jets is a matter of active debate. One particularly well-suited source for determining the location is the nearby bright radio galaxy 3C 84, harbouring a powerful jet. We investigated the origin of γ-rays that were measured during a recent γ-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 almost unpolarised overall, we find that close in time to the γ-ray flare peak regions at parsec-scale distances from the central engine, the linear polarisation increases fractionally. Under the physically well-motivated assumption of a causal relation between this polarisation enhancement and the γ-ray flare, and combined with insights from concurrent X-ray polarisation measurements, a physically motivated scenario is that the γ-rays are created in this region, in a process consistent with the synchrotron self-Compton mechanism.