Work

Ring of fire orphan

Year: 2026

Type: article

Abstract: We present 43 GHz very long baseline interferometry (VLBI) observations of the radio galaxy 3C 120 during its brightest γ-ray outburst (March 2018), which was recently associated with the IceCube neutrino alert IC-180213A. Despite reaching Lγ = 3.7 × 1044 erg s−1, contemporaneous X-ray monitoring from INTEGRAL/ISGRI, MAXI/GSC, and Swift/XRT revealed no variability across 0.3−200 keV, nor in the B, V, R, and I band optical observations or 37 & 235 GHz observations. Thus, it was designated an orphan flare. High-cadence VLBI imaging identified a new jet disturbance (N) propagating at βapp = (2.8 ± 1.3) through quasi-stationary features C1–C3. The γ-ray peak coincided spatially and temporally with N crossing C3 (r ∼ 0.38 mas), where we measured an increase (by a factor of 5) in the fractional polarization (m = 16%) and Δχ ∼ 24° electric vector position angle (EVPA) rotation, indicating localized magnetic field compression. The extreme Compton dominance (Lγ/Lsyn, blob ≈ 160) can naturally be explained by the ring of fire scenario, in which N (Γblob = 6, Bblob = 0.023 G) inverse-Compton scattering distributes synchrotron photons from C3, reproducing the observed γ-ray luminosity for physically reasonable parameters. We ruled out alternative mechanisms: spine-sheath geometric boosting requires implausibly large viewing-angle swings, while magnetic reconnection predicts excessive synchrotron emission and blob-star collisions cannot explain the extended (∼100−150 day) period of elevated activity. Unlike the 2014−2015 orphan flares attributed to rapid spine reorientation near the BLR, the 2018 event represents a distinct physical mechanism, namely, a propagating disturbance interacting with a stationary jet structure at about ten times the BLR radius. This work provides the first direct observational link between VLBI-resolved jet dynamics and orphan γ-ray emission in a radio galaxy.

Source: Springer Link (Chiba Institute of Technology)

DOI: 10.1051/0004-6361/202659357/pdf