Revealing a ribbon-like jet in OJ 287 with RadioAstron

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
² Interdisziplinäres Zentrum für Wissenschaftliches Rechnen (IWR), Universität Heidelberg, Im Neuenheimer Feld 205, 69120 Heidelberg, Germany
³ Korea Astronomy and Space Science Institute, Daedeok-daero 776, Yuseong-gu, Daejeon 34055, Republic of Korea
⁴ Department of Astronomy, Yonsei University, Yonsei-ro 50, Seodaemun-gu, 03722 Seoul, Republic of Korea
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁶ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁷ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, Roma 00133, Italy
⁸ Institut de Radioastronomie Millimétrique, Avenida Divina Pastora, 7, Local 20, E18012 Granada, Spain
⁹ Department of Physics and Astronomy, West Virginia University, PO Box 6315 Morgantown, WV 26506, USA
¹⁰ Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA
¹¹ Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands
¹² Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
¹³ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Rd., Shanghai 200030, China
¹⁴ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
¹⁵ Crimean Astrophysical Observatory, 298409 Nauchny, Crimea
¹⁶ Institute for Nuclear Research of the Russian Academy of Sciences, 60th October Anniversary Prospect 7a, Moscow 117312, Russia
¹⁷ Department of Astronomy, University of Illinois Urbana-Champaign, 1002 W. Green Street, Urbana, IL 61801, USA
¹⁸ Princeton Gravity Initiative, Princeton University, Princeton, NJ 08540, USA

^⋆ Corresponding author: traianouthalia@gmail.com

Received: 1 April 2025
Accepted: 6 June 2025

Abstract

We present space-based very long baseline interferometry observations of the BL Lac type object OJ 287 taken with RadioAstron at 22 GHz on April 25, 2016, in conjunction with a ground array comprising 27 radio telescopes. We detect ground-space fringes at projected baselines extending up to 4.6 Earth diameters, which allowed us to image the jet in OJ 287 with an angular resolution of ∼47 μas. Applying an advanced regularized maximum likelihood imaging method, we resolved the innermost jet structure with a complex morphology at a resolution of ∼15 μas (∼0.1 pc projected distance). For the first time, due to a favorable geometrical position of the jet in tandem with high data quality, we detect multiple sharp bends that form a “ribbon-like” jet structure that extends down to 1 mas. Two-dimensional Gaussian model-fitting reveals regions of the jet with brightness temperatures of more than 10¹³ K, indicative of strong Doppler boosting. Polarimetric imaging reveals that the electric vector position angles are predominantly perpendicular to the innermost jet direction, implying a dominant poloidal magnetic field component near the central engine. Complementary multi-epoch Very Long Baseline Array observations at 43 GHz provide a multifrequency view of the jet evolution. Ridgeline analysis of the 43 GHz data shows significant variations in the jet position angle from 2014 to 2017, behavior consistent with a rotating helical jet structure. Finally, we confirm the emergence of a new jet component (B15 or K), which may be associated with the source’s first TeV flare, and offer new observational constraints relevant to models involving a supermassive black hole binary.