RadioAstron Space VLBI Imaging of the Jet in M87. I. Detection of High Brightness Temperature at 22 GHz

Abstract

We present results from the first 22 GHz space very long baseline interferometric (VLBI) imaging observations of M87 by RadioAstron. As a part of the Nearby AGN Key Science Program, the source was observed in 2014 February at 22 GHz with 21 ground stations, reaching projected (u, v) spacings up to similar to 11 G.. The imaging experiment was complemented by snapshot RadioAstron data of M87 obtained during 2013-2016 from the AGN Survey Key Science Program. Their longest baselines extend up to similar to 25 G.. For all of these measurements, fringes are detected only up to similar to 2.8 Earth diameter or similar to 3G. baseline lengths, resulting in a new image with angular resolution of similar to 150 mu as or similar to 20 Schwarzschild radii spatial resolution. The new image not only shows edgebrightened jet and counterjet structures down to submilliarcsecond scales but also clearly resolves the VLBI core region. While the overall size of the core is comparable to those reported in the literature, the ground-space fringe detection and slightly superresolved RadioAstron image suggest the presence of substructures in the nucleus, whose minimum brightness temperature exceeds TB, min 10 similar to 12 K. It is challenging to explain the origin of this record-high TB, min value for M87 by pure Doppler boosting effect with a simple conical jet geometry and known jet speed. Therefore, this can be evidence for more extreme Doppler boosting due to a blazar-like small jet viewing angle or highly efficient particle acceleration processes occurring already at the base of the outflow.