Revisiting 3C 279 jet morphology with space VLBI at 26 microarcsecond resolution

TL;DR

This study utilizes space VLBI with RadioAstron at 22 GHz to image the blazar 3C 279 with ~26 μas resolution, aiming to resolve fine jet structure and magnetic-field configurations. The team applies regularized maximum likelihood imaging and closure data to reconstruct high-fidelity total-intensity and polarization maps, comparing the 2018 image to the 2014 RadioAstron results and BEAM-ME 43 GHz data. The 2018 observations reveal a core-dominated, straight jet with EVPAs aligned along the jet, indicating a predominantly toroidal or helical magnetic field, but no filamentary structure is detected as in 2014; synthetic-data tests suggest the non-detection could stem from uv-coverage and lower source brightness. The core exhibits a high observed brightness temperature of $T_{b,obs} \sim 1.6\times10^{12}$ K, consistent with near equipartition after Doppler correction, yielding $B \approx 0.2$ G; gamma-ray activity around the epoch shows no clear radio counterpart, highlighting the complex, multi-band behavior of this source. Overall, the results emphasize how uv-coverage and source brightness evolution influence the detectability of fine jet substructure and provide constraints on jet magnetic-field configuration in 3C 279.

Abstract

We present observations of the blazar 3C 279 at 22 GHz using the space VLBI mission RadioAstron on 2018 January 15. Images in both total intensity and fractional polarization are reconstructed using RML method implemented in the eht-imaging library. The electric vector position angles are found to be mostly aligned with the general jet direction, suggesting a predominantly toroidal magnetic field, in agreement with the presence of a helical magnetic field. Ground-space fringes were detected up to a projected baseline length of $\sim 8$G$λ$, achieving the angular resolution of around 26$μ$as. The fine-scale structure of the relativistic jet is found in our study extending to a projected distance of $\sim 180$ parsec from the radio core. However, the filamentary structure reported by previous RadioAstron observations of 2014 is not detected in our current study. We discuss potential causes for this phenomenon, together with a comparison using public 43 GHz data from the BEAM-ME program, showing a significant drop in the jet's total intensity. The optically thick core is observed with a brightness temperature of $ 1.6 \times 10^{12}$ K, consistent with equipartition between the energy densities of the relativistic particles and the magnetic field. This yields an estimated magnetic field strength of 0.2 G.

Figures (10)

• Figure 1: Coverage in the uv-plane of the fringe-fitted interferometric visibilities of 3C 279, observed by RadioAstron at 22 GHz on 2018 January 15. Top panel shows only the ground baselines, while bottom panel shows all baselines, with RadioAstron ones in grey.
• Figure 2: Jet structure of 3C 279 revealed by RadioAstron. a: Total intensity (left) and linearly polarized (right) RadioAstron image at 22 GHz obtained on 2018 January 15. Both images show brightness temperature in color scale (afmhot and grey scale, respectively), though the image on the right also shows the recovered electric vector position angles overlaid as ticks. Their length and color are proportional to the level of linearly polarized intensity and fractional polarization, respectively. EHT aligned image from panel b is also displayed in contours over total intensity image. b: The EHT image at 230 GHz obtained in April 2017 at 1:1 scale and aligned to our RadioAstron image -in contours- using their respective pixel with maximum brightness. Contours are equally spaced in 7 levels within the data range. c The 43 GHz close-in-time image from BEAM-ME program from Feurbary 2018. White ellipses at the bottom-left corners show the convolving beams. Bottom colour bars refer only to information displayed on a.
• Figure 3: Main image: a direct comparison between the 2014 and 2018 images of 3C 279 at 22 GHz with RadioAstron. The 2018 epoch is plotted in warm color scale afmhot and aligned with the 2014 epoch Fuentes_2023, in blue, using the brightest pixel. Top right image shows overlapped the Gaussian components from the model fitting process. The fifth component (E) is not shown since it is a large component necessary to take into account the diffuse emission.
• Figure 4: Gamma-ray light curve of 3C 279 (positionally associated with 4FGL J1256.1-0547) constructed from Fermi-LAT data using adaptive binning Lott12 with a constant relative uncertainty on flux of $\sim$15% in each bin. The vertical red dashed line indicates the epoch of RadioAstron observations on 15 January 2018, which coincides with the rising phase of a prominent short-duration (6.8 days) flare reaching a peak of $4.0\cdot10^{-5}$ ph cm$^{-2}$ s$^{-1}$ ($2.4\cdot10^{-8}$ erg cm$^{-2}$ s$^{-1}$) on January 18, 2018
• Figure 5: Synthetic data test using different data terms to probe filament detection using the RadioAstron$uv$-coverage of 2018 (first row) and BEAM-ME February 2018 (second row). Main images have a field of view of 1.1 $\times$ 1.1 mas, while miniatures at the top right show the main images convolved with its respective nominal beams on a field of view of 1.3 $\times$ 1.3 mas and 2 $\times$ 2 mas, for each row respectively. Units of the color wedges indicate brightness temperature in Kelvin. The nominal beam for each dataset is plotted in white at the bottom left of the miniature images.