---
title: 'TXS 2013+370: Gamma-Ray Blazar Insights'
url: https://www.emergentmind.com/topics/txs-2013-370
type: topic
---

# TXS 2013+370: Gamma-Ray Blazar Insights

TXS 2013+370 is a γ-ray-loud blazar at redshift $z=0.859$, notable for its exceptional multiwavelength variability and highly relativistic jet. As an FSRQ exhibiting prominent radio and γ-ray outbursts, TXS 2013+370 has served as a key laboratory for probing the innermost structures and physical processes governing blazar emission. Multi-epoch very long baseline interferometry (VLBI) imaging and time-resolved polarimetry, in combination with coordinated single-dish radio and Fermi-LAT $\gamma$-ray monitoring, have enabled the localization of the high-energy dissipation zone, characterization of jet kinematics and geometry on sub-parsec to tens-of-parsec scales, and investigation of the relation between high-energy flares and jet activity [1912.00776, 2511.15601].

## 1. VLBI Imaging and Jet Morphology

High angular resolution VLBI imaging at 22, 43, and 86 GHz using the RadioAstron space antenna, ground-based arrays (VLBA, GMVA, Effelsberg, Yebes, Onsala, Green Bank, Jodrell Bank), and coordinated single-dish facilities (OVRO 40m, SMA) has resolved the inner jet of TXS 2013+370 with a linear resolution as fine as $\sim0.4$ pc. The observed morphology is characterized by a bright, unresolved radio core at the jet apex, followed by a gently bending jet hosting both moving and stationary components. During the 2020–2021 GeV outburst, new VLBI images captured a compact near-core knot (“N2”) at $r \simeq 40$–$60~\mu\mathrm{as}$ from the core concurrent with the flare, while stationary features A2 and C3 were recovered further downstream [2511.15601]. The core maintains a flat, partially self-absorbed spectrum with $\langle \alpha_{\text{core}} \rangle \gtrsim -0.5$ across 22–86 GHz during major flares, indicating ongoing particle acceleration.

## 2. Jet Kinematics and Component Ejection

Model-fitting of VLBI visibilities with circular Gaussians has enabled tracking of distinct jet components epoch-by-epoch. Proper motions measured from the time evolution of feature-core separations yield apparent speeds as high as $\beta_{\text{app}}=13.9\pm0.9$ (component C2), with typical inner-jet speeds of $\beta_{\text{app}}=4.2\pm0.5$ (component A1, knot N2) and slower, quasi-stationary features at larger radii. Ejection epochs back-extrapolated for superluminal features align with multiwavelength flare timings, supporting a causal link between new component emergence and enhanced high-energy activity. The maximum observed speed sets a minimum bulk Lorentz factor $\Gamma_{\text{min}} \gtrsim 14$ and critical viewing angle $\theta_c \simeq 4.1^\circ$; inner-jet values near the core correspond to $\Gamma \sim 6$–$10$ with Doppler factors $\delta \lesssim 10$ for $\theta \sim \theta_c$ [1912.00776].

## 3. Jet Geometry and Expansion Profile

Comprehensive analysis of stacked VLBI maps reveals that the jet width $W(r)$ evolves according to a two-zone law: within $r\simeq 0.1$–$0.5$ mas ($\sim$1–4 pc projected), $W(r) \propto r^{0.49\pm0.04}$, consistent with parabolic collimation; beyond $r\gtrsim 0.4$–$0.5$ mas, a conical expansion with $W(r) \propto r^{1.02\pm0.01}$ dominates. The parabolic-to-conical transition occurs at a deprojected distance $\sim54$ pc ($\sim$1.5$\times$10$^6$ Schwarzschild radii for $M_{\rm BH}=4\times10^8\,M_\odot$). Extrapolating the inner parabolic law to zero width and matching to the 86 GHz core FWHM locates the radio core at $\leq 2.05\pm0.97$ pc downstream of the true jet apex [1912.00776].

## 4. Polarization Structure and Faraday Rotation

Full-polarization VLBI established that the core exhibits significant linear polarization ($m=3.3$–$4.5\%$) with a pronounced $\sim50^\circ$ rotation in the electric vector position angle (EVPA) between 43 and 86 GHz. Modeling the EVPA as a function of wavelength squared yields a uniform, extremely high rotation measure ${\rm RM} = (7.8\pm0.2)\times 10^4~\mathrm{rad~m}^{-2}$, unmatched by most blazar cores. This uniform RM, observed in both pixel- and integrated-Stokes analyses, indicates the presence of a highly magnetized, external Faraday screen, plausibly associated with circumnuclear material or the Galactic foreground [2511.15601]. This stable RM signature persists through major flaring episodes, suggesting the Faraday screen location is not altered by rapid inner-jet events.

## 5. Radio–Gamma-Ray Temporal Correlations

Correlated radio and γ-ray activity is a hallmark of TXS 2013+370. Discrete cross-correlation of Fermi-LAT (0.1–300 GeV or 0.1–100 GeV) and SMA 235 GHz or OVRO 15 GHz light curves consistently reveals that γ-ray events lead millimeter and centimeter radio flares, establishing a temporal lag: $\Delta t_{\gamma-235} = 49\pm30$ days and $\Delta t_{\gamma-15} = 102\pm12$ days, respectively. Using jet kinematics ($\beta_{\text{app}}=4.2$, $\theta=4.1^\circ$), these lags translate to deprojected site separations of $\Delta r_{\gamma-235}=1.3\pm0.81$ pc and $\Delta r_{\gamma-15}=2.71\pm0.47$ pc [1912.00776, 2511.15601]. Variability in the radio–γ-ray lag between epochs is attributed not to changes in the high-energy dissipation site, but instead to core-shift effects driven by changing opacity, resulting in the effective radio core moving downstream during flares while the true emission site remains spatially stable.

## 6. Localization of the Gamma-Ray Emission Region

Combining core–apex offsets with deprojected γ–radio separations confines the γ-ray production site to within the innermost parsec of the jet. For the 2021 outburst, the GeV emission zone lies between the jet apex and $\sim0.42$ pc downstream, consistent within $1\sigma$ uncertainties with both the broad-line region (BLR, $R_{\rm BLR}\sim0.07$ pc) and the innermost edge of the dusty torus. This constrains viable external Compton (EC) seed photon scenarios to: (a) optical/UV photons from the BLR, or (b) infrared photons from hot dust in the torus. No evidence requires moving the dissipation zone far downstream across flares; instead, radiative delays are regulated by synchrotron opacity and associated core shifts [2511.15601].

## 7. Physical Interpretation and Implications

Multi-band VLBI, polarimetric, and cross-correlation data for TXS 2013+370 collectively support a leptonic EC scenario in which relativistic electrons, energized by shocks or magnetic reconnection within the magnetically dominated, accelerating jet zone, upscatter either BLR or torus photons to γ-ray energies. The observed flat spectrum and emergence of localized jet knots during flares demonstrate ongoing particle acceleration. RM and polarization properties indicate a persistently ordered, strongly magnetized circumnuclear environment. The observed jet opening angle ($\varphi\sim1$–$2^\circ$) and bulk Lorentz factor ($\Gamma\sim10$) satisfy $\Gamma\varphi\sim0.3$ rad, in agreement with expectations from MHD jet models. The constancy of the γ-ray emission region across multiple flaring episodes indicates spatial stability of the high-energy dissipation process, modulated by core-shift–induced opacity variations rather than by migration of the radiative zone. This underscores the utility of multi-frequency, polarimetric VLBI for disentangling spatial from temporal effects in the high-energy physics of blazars [1912.00776, 2511.15601].

Source: https://www.emergentmind.com/topics/txs-2013-370