Catching the 2021 {\gamma}-ray flare in the blazar TXS 2013+370

Abstract

The $\gamma$-ray-loud blazar TXS 2013+370, a powerful multiwavelength emitter at $z = 0.859$, underwent an exceptional GeV outburst in late 2020-early 2021. In this work, we present full-polarization VLBI imaging at 22, 43, and 86 GHz together with contemporaneous single-dish monitoring (radio and $\gamma$-rays) to localize the high-energy dissipation site and probe the inner-jet structure and magnetic field topology. The images revealed a compact near-core knot at $r \simeq 40$-$60~\mu\mathrm{as}$ contemporaneously with the GeV flare and a flat, core-dominated spectrum ($\alpha \gtrsim -0.5$). The core has strong linear polarization and exhibits a $\sim 50^{\circ}$ EVPA rotation at 86 GHz; pixel-based and integrated fits yield a high, uniform rotation measure, ${\rm RM} = (7.8 \pm 0.2) \times 10^{4}~{\rm rad~m^{-2}}$, consistent with an external Faraday screen. Cross-correlation of Fermi-LAT and 15 GHz data shows a significant peak with the $\gamma$ rays leading by $\Delta t = (102 \pm 12)\,\mathrm{d}$; adopting $\beta_{\rm app} = 4.2 \pm 0.5$ and $\theta = 4.1^{\circ} \pm 0.2^{\circ}$ implies a de-projected separation $\Delta r_{\gamma-15} = (2.71 \pm 0.47)\,\mathrm{pc}$ and locates the GeV emission between the jet apex and $\sim 0.42$ pc ($1\sigma$ range) downstream. Our results do not uniquely pinpoint the emission site; rather, they support two valid scenarios. The $\gamma$-ray production may occur within the BLR ($\sim 0.07$pc), where external-Compton scattering of optical/UV photons produces the $\gamma$ rays, or beyond the BLR, reaching $\sim 0.42$ pc ($1\sigma$) within the inner parsecs, where external-Compton scattering of dusty-torus infrared photons dominates. Both scenarios are compatible within the allowed range of emission distances, while opacity-driven core shifts modulate the observed radio-$\gamma$ delay without requiring large relocations of the dissipation zone.