Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron

IV. The quasar 3C 345 at 18 cm: Magnetic field structure and brightness temperature^⋆

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: fpoetzl@mpifr-bonn.mpg.de
² Moscow Institute of Physics and Technology, Institutsky Per. 9, Dolgoprudny, Moscow Region 141700, Russia
³ Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
⁴ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Rome, Italy
⁵ JIVE – Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingekoo, The Netherlands
⁶ Dept. of Astrodynamics and Space Missions, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
⁷ CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia
⁸ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁹ Astro Space Center of Lebedev Physical Institute, Profsoyuznaya St. 84/32, Moscow 117997, Russia
¹⁰ INAF Istituto di Radioastronomia, Via P. Gobetti 101, Bologna 40129, Italy
¹¹ Aalto University Department of Electronics and Nanoengineering, PL 15500, 00076 Aalto, Finland
¹² Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
¹³ Center for Data Intensive and Time Domain Astronomy, Department of Physics and Astronomy, Michigan State University, 567 Wilson Rd, East Lansing MI 48824, USA
¹⁴ Sternberg Astronomical Institute, Moscow State University, Universitetskii Pr. 13, 119992 Moscow, Russia

Received: 22 September 2020
Accepted: 28 January 2021

Abstract

Context. Supermassive black holes in the centres of radio-loud active galactic nuclei (AGN) can produce collimated relativistic outflows (jets). Magnetic fields are thought to play a key role in the formation and collimation of these jets, but the details are much debated.

Aims. We study the innermost jet morphology and magnetic field strength in the AGN 3C 345 with an unprecedented resolution using images obtained within the framework of the key science programme on AGN polarisation of the Space VLBI mission RadioAstron.

Methods. We observed the flat spectrum radio quasar 3C 345 at 1.6 GHz on 2016 March 30 with RadioAstron and 18 ground-based radio telescopes in full polarisation mode.

Results. Our images, in both total intensity and linear polarisation, reveal a complex jet structure at 300 μas angular resolution, corresponding to a projected linear scale of about 2 pc or a few thousand gravitational radii. We identify the synchrotron self-absorbed core at the jet base and find the brightest feature in the jet 1.5 mas downstream of the core. Several polarised components appear in the Space VLBI images that cannot be seen from ground array-only images. Except for the core, the electric vector position angles follow the local jet direction, suggesting a magnetic field perpendicular to the jet. This indicates the presence of plane perpendicular shocks in these regions. Additionally, we infer a minimum brightness temperature at the largest (u, v)-distances of 1.1 × 10¹² K in the source frame, which is above the inverse Compton limit and an order of magnitude larger than the equipartition value. This indicates locally efficient injection or re-acceleration of particles in the jet to counter the inverse Compton cooling or the geometry of the jet creates significant changes in the Doppler factor, which has to be > 11 to explain the high brightness temperatures.