RadioAstron reveals a spine-sheath jet structure in 3C 273

¹ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, 00133 Rome, Italy
e-mail: gabriele.bruni@inaf.it
² IAA – Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
³ MPIfR – Max Planck Institute for Radio Astronomy, Auf dem Hügel, 69, 53121 Bonn, Germany
⁴ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁵ Aalto University Department of Electronics and Nanoengineering, PL 15500, 00076 Aalto, Finland
⁶ Lebedev Physical Institute of the Russian Academy of Sciences, Leninsky Prospekt 53, 119991 Moscow, Russia
⁷ Moscow Institute of Physics and Technology, Institutsky Per. 9, Dolgoprudny 141700 Moscow Region, Russia
⁸ Departament d’Astronomia i Astrofísica, Universitat de Valéncia, C/ Dr. Moliner, 50, 46100 Burjassot, Valéncia, Spain
⁹ Observatori Astronòmic, Universitat de Valéncia, C/ Catedràtic José Beltran, 2, 46980 Paterna, Valéncia, Spain
¹⁰ Technische Universität Berlin, Institut für Geodäsie und Geoinformationstechnik, Fakultät VI, Sekr. KAI 2-2, Kaiserin-Augusta-Allee 104-106, 10553 Berlin, Germany
¹¹ GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
¹² CSIRO Astronomy and Space Science, PO Box 76 Epping, NSW 1710, Australia
¹³ JIVE – Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingekoo, The Netherlands
¹⁴ Dept. of Astrodynamics and Space Missions, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
¹⁵ Crimean Astrophysical Observatory, Nauchny, 298409 Crimea
¹⁶ Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA
¹⁷ Sternberg Astronomical Institute, Moscow State University, Universitetskii Pr. 13, 119992 Moscow, Russia

Received: 14 September 2020
Accepted: 13 July 2021

Abstract

We present Space-VLBI RadioAstron observations at 1.6 GHz and 4.8 GHz of the flat spectrum radio quasar 3C 273, with detections on baselines up to 4.5 and 3.3 Earth Diameters, respectively. Achieving the best angular resolution at 1.6 GHz to date, we have imaged limb-brightening in the jet, not previously detected in this source. In contrast, at 4.8 GHz, we detected emission from a central stream of plasma, with a spatial distribution complementary to the limb-brightened emission, indicating an origin in the spine of the jet. While a stratification across the jet width in the flow density, internal energy, magnetic field, or bulk flow velocity are usually invoked to explain the limb-brightening, the different jet structure detected at the two frequencies probably requires a stratification in the emitting electron energy distribution. Future dedicated numerical simulations will allow the determination of which combination of physical parameters are needed to reproduce the spine-sheath structure observed by Space-VLBI with RadioAstron in 3C 273.