High-Resolution VLBI study of the γ-ray loud blazar TXS 2013+370

One of the long standing questions surrounding relativistic plasma jets, concerns the location and mechanism of the γ-ray emission. Some models suggest that the emission is located in sub-parsec scales from the central engine. The seed photons originate from the external photon-rich jet surroundings (External Compton), with possible reservoirs being, the accretion disk, the broad-line region (BLR) and the dusty torus. In contrast, some studies suggest that the γ-ray emission is placed in parsec scales where the seed photons may originate from the same synchrotron photons radiated by the jet (Synchrotron-Self-Compton). While the limited angular resolution of the current γ-ray detectors, prevent us to directly pinpoint the γ-ray production region, Very Long Baseline Interferometry (VLBI) can resolve jets down to event horizon scales and investigate the origin of γ-ray emission in blazars. A prominent candidate is the radio source TXS2013+370 at a redshift z = 0.859, that frequently undergoes γ-ray flaring episodes. On December 6, 2020, the source experienced a new γ-ray flare. Using the Very Long Baseline Array (VLBA) and the 100-m Effelsberg antenna at 22, 43, and 86 GHz we obtained total intensity and polarization data. The goal is to constrain the location of the γ-ray emission region in TXS 2013+370. After a VLBI calibration of the data, an imaging of the visibilities provided us with three total intensity maps. Via the model fitting technique, we parametrized the flux density distribution along the jet, using 2D circular Gaussian components. This allowed us to identify both the existing and a newly emerged jet component. Our polarization analysis revealed a core region dominated by linear polarized emission, showing a smooth EVPA rotation between 22, and 43 GHz, and a ~50o flip at 86 GHz close to the central engine, suggestive of a helical magnetic field structure. The Faraday rotation analysis showed a high Rotation Measurement (RM) reaching values up to 7.8∙104 rad/m2 with no exceptional gradients, indications which combined with the position of the source in the Galactic plane, suggest an external Faraday screen affecting the EVPA rotation. Complementing these polarization findings, our spectral index maps revealed a flat-spectrum core, consistent with its classification as a flat-spectrum radio quasar (FSRQ). Crucially, we constrained the location of the γ-ray emission. A DCCF analysis between the Fermi γ-ray and 15 GHz light curves, provided us with a time lag between the flares, Δt = ( 102 ± 3 ) days which was translated into a linear distance Δrγ-15 = ( 2.71 ± 0.34 ) pc. Using the R ≤ 2.05 ± 0.97 pc between the VLBI core and the jet apex, we estimated the linear distance of the γ-ray emission site downstream from the jet apex at Δr = ( 0.66 ± 1.03 ) pc. This places the emission region in sub-parsec to parsec scales from the BH with possible seed photon fields being the accretion disk, the BLR and the dusty torus. For TXS2013+370, with a BLR size ~0.07 pc the best candidate for EC is the dusty torus.