A Ring of Fire Orphan γ-Ray Flare in the Neutrino Candidate 3C 120

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 ${\beta }_{\mathrm{app}}=(2.8\pm 1.3)$ 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 ($L_{\gamma }/L_{\mathrm{syn},\mathrm{blob}}\approx 160$) is naturally explained by the Ring of Fire scenario, in which N (${\Gamma }_{\mathrm{blob}}=6$, $B_{\mathrm{blob}}=0.023$\,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 $\sim 10\times$ 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.

10 pages, 2 figures

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