Localizing the γ-ray emitting region in the blazar TXS 2013+370

Abstract

Aims. The γ-ray production mechanism and its localization in blazars are still a matter of debate. The main goal of this paper is to constrain the location of the high-energy emission in the blazar TXS 2013+370 and to study the physical and geometrical properties of the inner jet region on sub-pc scales. Methods. TXS 2013+370 was monitored during 2002-2013 with VLBI at 15, 22, 43, and 86 GHz, which allowed us to image the jet base with an angular resolution of ≥ 0.4 pc. By employing CLEAN imaging and Gaussian model-fitting, we performed a thorough kinematic analysis at multiple frequencies, which provided estimates of the jet speed, orientation, and component ejection times. Additionally, we studied the jet expansion profile and used the information on the jet geometry to estimate the location of the jet apex. VLBI data were combined with single-dish measurements to search for correlated activity between the radio, mm, and γ-ray emission. For this purpose, we employed a cross-correlation analysis, supported by several significance tests. Results. The high-resolution VLBI imaging revealed the existence of a spatially bent jet, described by co-existing moving emission features and stationary features. New jet features, labeled as A1, N, and N1, are observed to emerge from the core, accompanied by flaring activity in radio/mm-bands and γ rays. The analysis of the transverse jet width profile constrains the location of the mm core to lie ≤2 pc downstream of the jet apex, and also reveals the existence of a transition from parabolic to conical jet expansion at a distance of ∼54 pc from the core, corresponding to ∼1.5 × 10 6 Schwarzschild radii. The cross-correlation analysis of the broad-band variability reveals a strong correlation between the radio-mm and γ-ray data, with the 1 mm emission lagging ∼ 49 days behind the γ rays. Based on this, we infer that the high energy emission is produced at a distance of the order of ∼1 pc from the jet apex, suggesting that the seed photon fields for the external Compton mechanism originate either in the dusty torus or in the broad-line region.

Jet collimation in NGC 315 and other nearby AGN

“…In the LERG Centaurus A (Müller et al 2011), the jet was observed to be freely expanding already on scales of ∼0.1 pc, or ∼10 4 R S for M BH = 5 × 10 7 M (Neumayer 2010), in agreement with our findings for NGC 315 and other LERG. On the other hand, studies of the jet shape in some high redshift FSRQs (Algaba et al 2019;Traianou et al 2020) locate the shape transition at distances >10 6 R S , as found for the HEG in our sample.…”

“…Several observational studies have been performed with the aim of determining the extension of the acceleration and collimation region, thus providing constraints for theoretical models. Such studies were mostly focused on the analysis of low-luminosity radio galaxies, such as NGC 6251 (Tseng et al 2016), 3C 84 (Giovannini et al 2018), 3C 270 (Nakahara et al 2018), 3C 264 (Boccardi et al 2019), NGC 1052 (Nakahara et al 2020) and M 87 (Asada & Nakamura 2012;Mertens et al 2016;Hada et al 2016;Kim et al 2018;Nokhrina et al 2019), but some constraints for high-luminosity radio galaxies, like Cygnus A (Boccardi et al 2016a,b;Nakahara et al 2019), and blazars (Giroletti et al 2008;Akiyama et al 2018;Hada et al 2018;Algaba et al 2019;Traianou et al 2020) were also provided. Moreover, part of the MOJAVE sample was investigated by Pushkarev et al (2017), Kovalev et al (2020).…”

Jet collimation in NGC 315 and other nearby AGN

“…In the LERG Centaurus A (Müller et al 2011), the jet was observed to be freely expanding already on scales of ∼0.1 pc, or ∼10 4 R S for M BH = 5 × 10 7 M (Neumayer 2010), in agreement with our findings for NGC 315 and other LERG. On the other hand, studies of the jet shape in some high redshift FSRQs (Algaba et al 2019;Traianou et al 2020) locate the shape transition at distances > 10 6 R S , as found for the HEGs in our sample.…”

“…Several observational studies have been performed with the aim of determining the extension of the acceleration and collimation region, thus providing constraints for theoretical models. Such studies were mostly focused on the analysis of lowluminosity radio galaxies, such as NGC 6251 (Tseng et al 2016), 3C 84 (Giovannini et al 2018), 3C 270 (Nakahara et al 2018), 3C 264 (Boccardi et al 2019), NGC 1052 (Nakahara et al 2020) and M 87 (Asada & Nakamura 2012;Mertens et al 2016;Hada et al 2016;Kim et al 2018;Nokhrina et al 2019), but some constraints for high-luminosity radio galaxies, like Cygnus A (Boccardi et al 2016b,a;Nakahara et al 2019), and blazars (Giroletti et al 2008;Akiyama et al 2018;Hada et al 2018;Algaba et al 2019;Traianou et al 2020) were also provided. Moreover, part of the MOJAVE sample was investigated by Pushkarev et al (2017); Kovalev et al (2020).…”

Accretion mode and jet collimation in active galactic nuclei

“…Due to the small redshift cut, the sample in Boccardi et al (2021) did not include any Flat Spectrum Radio Quasars (FSRQ), forming the parent population of HERG in the unified scheme. However, some first results for this class of sources have been obtained: in TXS 2013 + 370 (Traianou et al 2020) and 4C 38.41 (Algaba et al 2019) a shape transition was detected at a distance through the analysis of cm‐ and mm‐VLBI data. The third new source included in Figure 2 is the LERG Centaurus A.…”