The jet of S5 0716+71 at μas scales with RadioAstron

Ground-space interferometer RadioAstron provides unique opportunity to probe detail structure of the distant active galactic nuclei at μas scales. Here we report on RadioAstron observations of the BL Lac object S5 0716 + 71, performed in a framework of the AGN Polarization and Survey Key Science Programs at 22 GHz during 2012–2018. We obtained the highest angular resolution image of the source to date, at [Formula presented]as. It reveals complex structure of the blazar jet in the inner 100 μas, with emission regions that can be responsible for the blazar variability at timescales of a few days to week. Linear polarization is detected in the core and jet areas at the projected baselines up to about 5.6 Earth diameters. The observed core brightness temperature in the source frame of ⩾2.2×1013 K is in excess of theoretical limits, suggesting the physical conditions are far from the equipartition between relativistic particles and magnetic field. © 2019 COSPAREVK acknowledges support from the Italian Space Agency under contract ASI-INAF 2015-023-R.O . JLG was supported by the Spanish Ministry of Economy and Competitiveness grants AYA2013-40825-P and AYA2016-80889-P . YYK was partly supported by the Russian Foundation for Basic Research (project 17-02-00197 ). The RadioAstron project is led by the Astro Space Center of the Lebedev Physical Institute of the Russian Academy of Sciences and the Lavochkin Scientific and Production Association under a contract with the State Space Corporation ROSCOSMOS, in collaboration with partner organizations in Russia and other countries. The European VLBI Network is a joint facility of independent European, African, Asian, and North American radio astronomy institutes. Scientific results from data presented in this publication are derived from the following EVN project code: GL041. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc