The γ-ray–loud blazar, a powerful multiwavelength emitter at z=0. 859, underwent an exceptional gigaelectron volt outburst in late 2020 to early 2021. In this work, we present full polarization VLBI imaging at 22, 43, and 86, GHz (11 February 2021) together with contemporaneous single-dish monitoring (OVRO 15, GHz; SMA 226, GHz) and TXS 2013+370 Fermi –LAT light curves to localize the high-energy dissipation site and probe the magnetic field of the inner jet. The images enabled us to study the jet structure and field topology on sub-parsec scales, revealing a compact near-core knot at r = (7. 8, consistent with an external Faraday screen. Performing a cross-correlation of –LAT and 15, GHz light curves revealed a highly significant peak, with the γ rays leading by Δ t= (102 =4. 2 ! implies a de-projected separation of Δ r_ μas along with the gigaelectron volt (GeV) flare and a flat core-dominated spectrum (α≳-0. 5). The core has strong linear polarization and exhibits a electric vector polarization angle rotation at 86, GHz. The pixel-based and integrated fits we employed yielded a high, uniform rotation measure, mathrm RM 4 -2 Fermi Adopting β_ ̊m app and þeta=4. 1^ ̧irc ̧irc γ-15 = (2. 71 and locates the GeV emission between the jet apex and sim0. 42, pc (in the 1σ range) downstream. Our results do not pinpoint the emission site; rather, they support two valid scenarios. The γ-ray production occurs within the broad-line region (sim0. 07, pc), where external-Compton scatters optical/UV photons to γ-rays, and beyond the broad-line region, reaching sim0. 42 pc (1σ) within the inner parsecs, where external-Compton scattering of dusty-torus infrared photons dominates. Both scenarios are compatible in the allowed range of emission distances, while opacity-driven core shifts modulate the observed radio–γ delay without requiring large relocations of the dissipation zone.
Traianou et al. (Fri,) studied this question.