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

¹ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, E-18008 Granada, Spain
² Max-Planck-Institut für RadioAstronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
³ INAF Istituto di RadioAstronomia, Via Gobetti 101, 40129 Bologna, Italy
⁴ Aalto University Department of Electronics and Nanoengineering, PL 15500 00076 Aalto, Germany
⁵ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁶ Crimean Astrophysical Observatory, 298409 Nauchny, Crimea
⁷ Astro Space Centre of Lebedev Physical Institute, Profsoyuznaya 84/32, Moscow, 117997, Russia
⁸ Interdisziplinäres Zentrum für Wissenschaftliches Rechnen (IWR), Universität Heidelberg, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
⁹ Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
¹⁰ Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
¹¹ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Rd., Shanghai 200030, China
¹² Institute of Astrophysics, Foundation for Research and Technology – Hellas, N. Plastira 100, Voutes, GR-70013 Heraklion, Greece
¹³ University of Crete, Department of Physics & Institute of Theoretical & Computational Physics, 70013 Heraklion, Greece
¹⁴ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla, 4059 Valparaíso, Chile

^★ Corresponding author: ttoscano@iaa.es

Received: 17 April 2025
Accepted: 3 October 2025

Abstract

We present observations of the blazar 3C 279 at 22 GHz by the space-based very long baseline interferometry mission RadioAstron from January 15, 2018. We reconstructed images in both total intensity and fractional polarization using the regularized maximum likelihood 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 and in agreement with the presence of a helical magnetic field. Ground-space fringes were detected up to a projected baseline length of ∼8 Gλ, achieving an angular resolution of around 26 μas. The fine-scale structure of the relativistic jet is found in our study to extend to a projected distance of ∼180 parsec from the radio core. However, the filamentary structure reported by previous RadioAstron observations from 2014 is not detected in our current study. We discuss potential causes for this phenomenon and present a comparison using public 43 GHz data from the BEAM-ME program showing a significant drop in the jet’s total intensity. We observe that the optically thick core has a brightness temperature of 1.6 × 10¹² 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.