A. P. Lobanov

We have used VLBA fringe visibility data obtained at 15 GHz to examine the compact structure in 250 extragalactic radio sources. With projected baselines out to 440 million wavelengths we are able to investigate source structure on typical angular scales as small as 0.05 mas. For 171 sources in our sample, more than half of the flux density seen by the VLBA remains unresolved on the longest baselines. The 163 sources in our list with median 15 GHz correlated flux density in excess of 0.5 Jy on these long baselines will be useful as fringe-finders for short wavelength VLBA observations. We have modeled the core of each source as an elliptical Gaussian component. For about 60% of the sources, at least at one epoch, the core component appears unresolved generally along a direction transverse to the jet direction. These core components are usually less than 0.05 mas in size. IDV sources show, in general, more compact and core dominant structure on sub-milliarcsecond scales that non IDV sources; IDVs with higher amplitude of intraday variations tend to have higher unresolved VLBA flux density. The most variable sources tend to have the most compact VLBA structure. The gamma-ray loud AGN appear to have more compact VLBA structure than the gamma-ray quiet AGN. This suggests that the mechanism of radio emission of compact components and the mechanism of gamma-ray emission are related. Typical estimates or lower limits to the brightness temperature of the jet cores are in the range of 10^11 to 10^13 K but extend up to 5x10^13 K, apparently in excess of the equipartition brightness temperature, or the inverse Compton limit for stationary synchrotron sources. The largest component speeds are observed only in radio sources with high observed brightness temperatures as would be expected from relativistic beaming (abridged).

We present measurements of the properties of the central radio source in M87 using Event Horizon Telescope data obtained during the 2017 campaign. We develop and fit geometric crescent models (asymmetric rings with interior brightness depressions) using two independent sampling algorithms that consider distinct representations of the visibility data. We show that the crescent family of models is statistically preferred over other comparably complex geometric models that we explore. We calibrate the geometric model parameters using general relativistic magnetohydrodynamic (GRMHD) models of the emission region and estimate physical properties of the source. We further fit images generated from GRMHD models directly to the data. We compare the derived emission region and black hole parameters from these analyses with those recovered from reconstructed images. There is a remarkable consistency among all methods and data sets. We find that >50% of the total flux at arcsecond scales comes from near the horizon, and that the emission is dramatically suppressed interior to this region by a factor >10, providing direct evidence of the predicted shadow of a black hole. Across all methods, we measure a crescent diameter of 423 as and constrain its fractional width to be <0.5. Associating the crescent feature with the emission surrounding the black hole shadow, we infer an angular gravitational radius of GM/Dc 2 =3.80.4 as. Folding in a distance measurement of -+ 16.8 Mpc 0.7 0.8 gives a black hole mass of = | | M M 6.5 0.2 0.7 10 stat sys 9

Aims. We have investigated a frequency-dependent shift in the absolute position of the optically thick apparent origin of parsec-scale jets (“core shift” effect) to probe physical conditions in ultra-compact relativistic outflows in active galactic nuclei.&#13;\nMethods. We used multi-frequency Very Long Baseline Array (VLBA) observations of 191 sources carried out in 12 epochs in 2006 within the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) program. The observations were performed at 8.1, 8.4, 12.1, and 15.4 GHz. We implemented a method of determining the core shift vector based on (i) image registration by two-dimensional normalized cross-correlation and (ii) model-fitting the source brightness distribution to take into account a non-zero core component offset from the phase center.&#13;\nResults. The 15.4−8.1, 15.4−8.4, and 15.4−12.1 GHz core shift vectors are derived for 163 sources, and have median values of 128, 125, and 88 μas, respectively, compared to the typical measured errors of 50, 51, 35 μas. The effect occurs predominantly along the jet direction, with departures smaller than 45° from the median jet position angle in over 80% of the cases. Despite the moderate ratio of the observed frequencies (&lt;2), core shifts significantly different from zero (&gt;2σ) are detected for about 55% of the sources. These shifts are even better aligned with the jet direction, deviating from the latter by less than 30° in over 90% of the cases. There is an indication that the core shift decreases with increasing redshift. Magnetic fields in the jet at a distance of 1 parsec from the central black hole, calculated from the obtained core shifts, are found to be systematically stronger in quasars (median B_1 ≈ 0.9 G) than those in BL Lacs (median B_1 ≈ 0.4 G). We also constrained the absolute distance of the core from the apex of the jet at 15 GHz as well as the magnetic field strength in the 15 GHz core region.

The Russian Academy of Sciences and Federal Space Agency, together with the participation of many international organizations, worked toward the launch of the RadioAstron orbiting space observatory with its onboard 10-m reflector radio telescope from the Baikonur cosmodrome on July 18, 2011. Together with some of the largest ground-based radio telescopes and a set of stations for tracking, collecting, and reducing the data obtained, this space radio telescope forms a multi-antenna ground-space radio interferometer with extremely long baselines, making it possible for the first time to study various objects in the Universe with angular resolutions a million times better than is possible with the human eye. The project is targeted at systematic studies of compact radio-emitting sources and their dynamics. Objects to be studied include supermassive black holes, accretion disks, and relativistic jets in active galactic nuclei, stellar-mass black holes, neutron stars and hypothetical quark stars, regions of formation of stars and planetary systems in our and other galaxies, interplanetary and interstellar plasma, and the gravitational field of the Earth. The results of ground-based and inflight tests of the space radio telescope carried out in both autonomous and ground-space interferometric regimes are reported. The derived characteristics are in agreement with the main requirements of the project. The astrophysical science program has begun.

Context. Very long baseline interferometry (VLBI) imaging of radio emission from extragalactic jets provides a unique probe of physical mechanisms governing the launching, acceleration, and collimation of relativistic outflows.