# The jet of S5 0716+71 at $\mu$as scales with RadioAstron

## TL;DR

RadioAstron observations of the blazar S5 0716+71 at unprecedented microarcsecond resolution reveal complex jet structures, high brightness temperatures, and polarization features, providing insights into the innermost regions of active galactic nuclei.

## Contribution

First high-resolution radio image of S5 0716+71 at 22 GHz with 57x24 μas resolution, unveiling detailed jet structure and polarization at microarcsecond scales.

## Key findings

- Detected complex jet structure within 100 μas
- Measured core brightness temperature exceeding theoretical limits
- Observed polarization up to 5.6 Earth diameters baseline

## Abstract

Ground-space interferometer RadioAstron provides unique opportunity to probe detail structure of the distant active galactic nuclei at $\mu$as 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 $57\times24 \mu$as. It reveals complex structure of the blazar jet in the inner 100 $\mu$as, 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 $\geq2.2\times10^{13}$ K is in excess of theoretical limits, suggesting the physical conditions are far from the equipartition between relativistic particles and magnetic field.