RadioAstron Early Science Program Space-VLBI AGN survey: strategy and first results

Abstract: RadioAstron is a project to use the 10m antenna on board the dedicated SPEKTR-R spacecraft, launched on 2011 July 18, to perform Very Long Baseline Interferometry from space - Space-VLBI. We describe the strategy and highlight the first results of a 92/18/6/1.35cm fringe survey of some of the brighter radio-loud Active Galactic Nuclei (AGN) at baselines up to 25 Earth diameters (D_E). The survey goals include a search for extreme brightness temperatures to resolve the Doppler factor crisis and to constrain possible mechanisms of AGN radio emission, studying the observed size distribution of the most compact features in AGN radio jets (with implications for their intrinsic structure and the properties of the scattering interstellar medium in our Galaxy) and selecting promising objects for detailed follow-up observations, including Space-VLBI imaging. Our survey target selection is based on the results of correlated visibility measurements at the longest ground-ground baselines from previous VLBI surveys. The current long-baseline fringe detections with RadioAstron include OJ 287 at 10 D_E (18cm), BL Lac at 10 D_E (6cm) and B0748+126 at 4.3 D_E (1.3 cm). The 18 and 6cm-band fringe detections at 10 D_E imply brightness temperatures of T_b ~ 10^13 K, about two orders of magnitude above the equipartition inverse Compton limit. These high values of T_b might indicate that the jet flow speed is often higher than the jet pattern speed.