Polarisation as a probe of neutrino emission from blazars

TL;DR

This study investigates the connection between jet shocks in blazars and neutrino emission, using polarimetric imaging to identify potential sites of proton acceleration during a flare event.

Contribution

It provides evidence linking jet shocks to neutrino production in blazars, highlighting the role of polarimetric VLBI imaging in identifying emission sites.

Findings

Shock fronts in the jet likely contributed to the flare.

Conditions for proton-photon interactions were present.

Neutrino emission could be linked to jet shock activity.

Abstract

The source of extragalactic neutrinos in the TeV-PeV range is a matter of very active research, with blazar jets having been postulated to be the origin of at least some of the detections. The blazar PKS 0735+178 is a prominent example; during its multi-band flare in late 2021 a neutrino event was reported by four observatories, with its origin consistent with the direction of that source. While no new jet component was observed to be ejected during that narrow time-frame, our analysis shows that a propagating shock front originating from the core region was the likely source of the multi-band flare, using very-long-baseline interferometry images of PKS 0735+178 in polarised light. Taken together, our findings are suggestive of a coherent scenario in which the shock may contribute to the acceleration of protons, with the target photons potentially originating either from the ambient medium surrounding the jet or from proton synchrotron radiation. The necessary conditions for neutrino emission via proton-photon interactions are, hence, present in this jet.