Probing the innermost regions of AGN jets and their magnetic fields with RadioAstron. III. Blazar S5 0716+71 at microarcsecond resolution

Abstract

We present RadioAstron Space VLBI imaging observations of the BL Lac object S5 0716+71 made on January 3-4 2015 at a frequency of 22 GHz (wavelength $\hat{I}»=1.3$ cm). The observations were made in the framework of the AGN Polarization Key Science Program. The source was detected on projected space-ground baselines up to 70 833 km (5.6 Earth diameters) for both, parallel hand and cross-hand interferometric visibilities. We have used these detections to obtain a full-polarimetric image of the blazar at an unprecedented angular resolution of 24 $\hat{I}¼$as, the highest for this source to date. This enabled us to estimate the size of the radio core to be $<12\times 5\hat{I}¼$as and to reveal a complex structure and a significant curvature of the blazar jet in the inner 100 $\hat{I}¼$as, which is an indication that the jet viewing angle lies inside the opening angle of the jet conical outflow. Fairly highly (15%) linearly polarized emission is detected in a jet region of 19 $\hat{I}¼$as in size, located 58 $\hat{I}¼$as downstream from the core. The highest brightness temperature in the source frame is estimated to be $>2.2\times 10^{13}$ K for the blazar core. This implies that the inverse Compton limit must be violated in the rest frame of the source, even for the largest Doppler factor $\hat{I}´\sim 25$ reported for 0716+714.

16 pages, 6 figures, 3 tables; accepted to ApJ