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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.