Observing the quasar 3C 345 at 18 cm with RadioAstron: Magnetic field structure and brightness temperature
Abstract
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. 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 program on AGN polarisation of the space VLBI mission RadioAstron. 3C 345 was observed at 1.6~GHz on March 30, 2016 with RadioAstron and 18 ground-based radio telescopes in full polarisation mode. Our images, in both total intensity and linear polarisation, reveal a complex jet structure at 300~$\mu$as 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. Based off earlier VLBI observations, we do not expect significant Faraday rotation in the source. Additionally, we infer a minimum brightness temperature at the largest $uv$-distances of $1.1\times10^{12}$~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, that has to be $>11$ to explain the high brightness temperatures.
- Publication:
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43rd COSPAR Scientific Assembly. Held 28 January - 4 February
- Pub Date:
- January 2021
- Bibcode:
- 2021cosp...43E1407P