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.
Highlights
A fraction of accreting supermassive black holes in the centres of active galactic nuclei (AGN) produce collimated relativistic outflows manifesting themselves through broadbandThe reduced images (FITS format) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/648/A82 continuum emission from the radio to gamma-ray regime
The improvement in angular resolution of the obtained image due to the participation of RadioAstron over the ground-only image is about a factor of 7 along the jet direction
– We present Space very long baselines interferometry (VLBI) images obtained with the RadioAstron mission in both total and linearly polarised intensity of the flat spectrum radio quasar (FSRQ) 3C 345 at 1.6 GHz with an angular resolution of ∼300 μas
Summary
A fraction of accreting supermassive black holes in the centres of active galactic nuclei (AGN) produce collimated relativistic outflows (jets) manifesting themselves through broadband. 4.1 in Bruni et al 2020, for the project description) aims to develop, commission, and exploit the unprecedented high angular resolution polarisation capabilities of RadioAstron to probe the innermost regions of AGN jets and their magnetic fields. With this it is possible to accurately determine potential Faraday rotation gradients (e.g. Gómez et al 2011; Zamaninasab et al 2013) at the most compact angular scales, revealing changes of the magnetic field within the jet.
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