Synchrotron Self-Absorption Spectral Modeling Reveals a Magnetically Driven Shock-in-Jet Scenario in Blazar 1156+295

Abstract

Unveiling the launching and driving mechanisms of powerful jets in active galactic nuclei (AGNs) is crucial for understanding the co-evolution of supermassive black holes (SMBHs) and their host galaxies. 1156+295 is a blazar at a redshift of z=0.729 and exhibits significant variability in long-term radio monitoring. Using multi-frequency Effelsberg single-dish flux density data from 2007 to 2012, we performed synchrotron self-absorption (SSA) spectral modeling and extracted the turnover frequency and turnover flux density. By combining SSA spectral modeling with the core size and brightness temperature from quasi-simultaneous very long baseline interferometry (VLBI) images, we estimated the jet magnetic-field strength and magnetic flux, and investigated their temporal evolution in 1156+295. The evolution of radio flux density, spectral shape, and jet structure is consistent with the shock-in-jet framework. The inferred magnetic flux reaching or exceeding the magnetically arrested disk (MAD) threshold, together with evidence that magnetic energy release precedes the radio flares, supports a magnetically driven jet scenario. Overall, our results place magnetic-field measurements, spectral evolution, and inner-jet structural changes on a common timeline, providing observational constraints on their coupled evolution during flares.

Figures (7)

• Figure 1: Top and middle panels: Radio light curves of 1156+295 constructed using VLBI (top) and Effelsberg single-dish data (middle) during 2007--2012. The same symbols used to denote VLBI bands are applied to the corresponding frequencies on the Effelsberg light curves. Bottom panel: VLBI-to-single-dish flux density ratio at matched epochs.
• Figure 2: The VLBI images at 43 GHz of the blazar 1156+295 convolved with the same beam of $0.52\,\mathrm{mas} \times 0.29\,\mathrm{mas}$ (FWHM) at major axis position angle $29.0^{\circ}$ (dashed ellipse at the lower left) between 2009 April 1 and 2010 April 10. The residual RMS noise is $\sigma=2.34$ mJy beam$^{-1}$, and the contours are drawn at $[-1, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512] \times 3\sigma$. The red circles/ellipses represent the positions and sizes of the fitted Gaussian models for the core (central) and jet (upper) components. The horizontal and inclined light blue dotted lines represent the reference position of the core and the fitted trajectory of the moving jet component, respectively. Linear regression provides an apparent jet proper motion of $(0.287 \pm 0.015)$ mas year$^{-1}$ with a coefficient of determination $R^2=0.97$.
• Figure 3: Selected representative radio spectra during the 2007--2012 evolution
• Figure 4: Turnover flux density $S_{\rm m}$ vs. turnover frequency $\nu_{\rm m}$ for 1156+295 during 2009--2012.
• Figure 5: Parameters and results of the magnetic field measurements. The downward-pointing triangles represent upper limits. The horizontal dashed line in the bottom panel indicates the predicted MAD magnetic flux of $\Phi_{\rm MAD}=3.13\times10^{33}$ G cm$^2$.