A Ring of Fire Orphan {\gamma}-Ray Flare in the Neutrino Candidate 3C 120

TL;DR

This study links VLBI-resolved jet dynamics with an orphan gamma-ray flare in 3C 120, supporting the Ring of Fire model where jet disturbances inverse-Compton scatter synchrotron photons.

Contribution

First direct observational evidence connecting jet disturbances with orphan gamma-ray flares in a radio galaxy, validating the Ring of Fire scenario.

Findings

Gamma-ray peak coincided with a jet disturbance crossing a stationary feature.

Significant polarization increase and EVPA rotation at the flare site.

The Ring of Fire model explains the high Compton dominance with reasonable jet parameters.

Abstract

We present 43\,GHz VLBI observations of the radio galaxy 3C~120 during its brightest $\gamma$-ray outburst (March 2018), recently associated with the IceCube neutrino alert IC-180213A. Despite reaching $L_\gamma = 3.7 \times 10^{44}$\,erg\,s$^{-1}$, contemporaneous X-ray monitoring from INTEGRAL/ISGRI, MAXI/GSC, and \textit{Swift}/XRT revealed no variability across 0.3-200\,keV, nor in B, V, R, and I band optical observations or 37 \& 235\,GHz observations, establishing an orphan flare. High-cadence VLBI imaging identified a new jet disturbance (N) propagating at $\beta_{\rm app} = (2.8 \pm 1.3)$ through quasi-stationary features C1-C3. The $\gamma$-ray peak coincided spatially and temporally with N crossing C3 ($r \sim 0.38$\,mas), where we measured a factor-of-5 increase in fractional polarization ($m = 16\%$) and $\Delta\chi \sim 24^\circ$ EVPA rotation, indicating localized magnetic field compression. The extreme Compton dominance ($L_\gamma / L_{\rm syn,blob} \approx 160$) is naturally explained by the Ring of Fire scenario, in which N ($\Gamma_{\rm blob} = 6$, $B_{\rm blob} = 0.023$\,G) inverse-Compton scatters synchrotron photons from C3, reproducing the observed $\gamma$-ray luminosity for physically reasonable parameters. Unlike the 2014-2015 orphan flares attributed to rapid spine reorientation near the BLR, the 2018 event represents a distinct physical mechanism, a propagating disturbance interacting with stationary jet structure at $\sim10\times$ the BLR radius.This work provides the first direct observational link between VLBI-resolved jet dynamics and orphan $\gamma$-ray emission in a radio galaxy.