Inverse Compton cooling limits the brightness temperature of the radiating plasma to a maximum of 1011.5 K. Relativistic boosting can increase its observed value, but apparent brightness temperatures much in excess of 1013 K are inaccessible using ground-based very long baseline interferometry (VLBI) at any wavelength. We present observations of the quasar 3C 273, made with the space VLBI mission RadioAstron on baselines up to 171,000 km, which directly reveal the presence of angular structure as small as 26 μas (2.7 light months) and brightness temperature in excess of 1013 K. These measurements challenge our understanding of the non-thermal continuum emission in the vicinity of supermassive black holes and require a much higher Doppler factor than what is determined from jet apparent kinematics.

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.

We present astrometric results for compact extragalactic objects observed with the Very Long Baseline Array at radio frequencies of 24 and 43 GHz. Data were obtained from ten 24 hr observing sessions made over a five-year period. These observations were motivated by the need to extend the International Celestial Reference Frame (ICRF) to higher radio frequencies to enable improved deep space navigation after 2016 and to improve state-of-the-art astrometry. Source coordinates for 268 sources were estimated at 24 GHz and for 131 sources at 43 GHz. The median formal uncertainties of right ascension and declination at 24 GHz are 0.08 and 0.15 mas, respectively. Median formal uncertainties at 43 GHz are 0.20 and 0.35 mas, respectively. Weighted root-mean-square differences between the 24 and 43 GHz positions and astrometric positions based on simultaneous 2.3 and 8.4 GHz Very Long Baseline Interferometry observations, such as the ICRF, are less than about 0.3 mas in both coordinates. With observations over five years we have achieved a precision at 24 GHz approaching that of the ICRF but unaccounted systematic errors limit the overall accuracy of the catalogs.

We have measured the sub-milli-arcsecond structure of 274 extragalactic sources at 24 and 43 GHz in order to assess their astrometric suitability for use in a high frequency celestial reference frame (CRF). Ten sessions of observations with the Very Long Baseline Array have been conducted over the course of $\sim$5 years, with a total of 1339 images produced for the 274 sources. There are several quantities that can be used to characterize the impact of intrinsic source structure on astrometric observations including the source flux density, the flux density variability, the source structure index, the source compactness, and the compactness variability. A detailed analysis of these imaging quantities shows that (1) our selection of compact sources from 8.4 GHz catalogs yielded sources with flux densities, averaged over the sessions in which each source was observed, of about 1 Jy at both 24 and 43 GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25% relative to their mean values, with variations in the session-to-session flux density scale being less than 10%, (3) sources were found to be more compact with less intrinsic structure at higher frequencies, and (4) variations of the core radio emission relative to the total flux density of the source are less than 8% on average at 24 GHz. We conclude that the reduction in the effects due to source structure gained by observing at higher frequencies will result in an improved CRF and a pool of high-quality fiducial reference points for use in spacecraft navigation over the next decade.

Submitted by Alan Marscher & Svetlana Jorstad (Boston U.), David Murphy, David Meier, Robert Preston, & Stephen Unwin (JPL), Kenneth Kellermann, Joan Wrobel, & Jonathan Romney (NRAO), Daniel Homan (Denison U.), Matthew Lister (Purdue U.), Glenn Piner (Whittier College), Lincoln Greenhill & Mark Reid (SAO), Gregory Taylor (U. New Mexico), Ann Wehrle (Space Science Institute), David Roberts (Brandeis U.), Anthony Readhead (Caltech), Markus Bottcher (Ohio U.), Markos Georganopoulos (U. Maryland), Steven Bloom (HampdenSydney Col.), Kenneth Johnston (US Naval Obs.), C.C. Cheung (NASA/GSFC), Thomas Krichbaum (Max-Planck-Institut fur Radioastronomie), M. Tsuboi (ISAS/JAXA, Japan), M. Inoue (NAOJ, Japan), S. Kameno (Kagoshima U., Japan)

We present VLBA observations of the Zeeman effect in H(2)O masers in the high-mass star-forming region OH 43.8-0.1, where we observed 116 maser features. These masers may be arranged in several groups: the most prominent are an arc-shaped structure to the north, a central cluster, two groups located symmetrically around the central cluster to its northeast and southwest, and a group in the extreme south. The highest velocity (redshifted) masers are in the center of the northern arc. The observed morphology of masers in OH 43.8-0.1 suggests a stellar object ( or objects) located within the central cluster of masers, driving outflows to the north and south; the redshifted and blueshifted group in the northern arc may represent the leading edge of two or more such outflows. The two groups located symmetrically around the central cluster may suggest a circumstellar disk of diameter 3000 AU. Seven masers in OH 43.8-0.1 are above our Zeeman detection limit. We detected magnetic fields in the range 10-20 mG in four of these masers and imposed sensitive upper limits on the other three. Three detections are for masers in the northern arc; the fourth is in the central cluster. We find no significant difference between the magnetic field strengths in these two groups. In the northern arc we detect a magnetic field reversal over a scale as small as 170 AU. We use our Zeeman-effect results to examine connections between the pre- and postshock magnetic fields and densities. The predicted preshock magnetic field strength and density are consistent with the fields and densities observed in typical preshock regions. The predicted postshock density also appears to be in the regime for optimal H(2)O maser pumping. Finally, we find that the magnetic and kinetic energy densities are likely in equilibrium in both pre- and postshock regions, meaning that the magnetic field must affect significantly the outflow dynamics.

We present phase-referenced VLBI results on the radio continuum and the OH 18 cm megamaser emission from the ultraluminous infrared galaxy, IRAS 17208-0014. The observations were carried out at 1599 MHz using the Very Long Baseline Array, the phased VLA, and the Green Bank Telescope. The highest resolution radio continuum results show several compact sources with brightness temperatures on the order of 106 K. These sources are more likely to be clustered supernova remnants and/or luminous radio supernovae. However, the agreement between the number of observed and expected compact sources above the 5 sigma level supports the possibility that each one of the compact sources could be dominated by a recently detonated luminous radio supernova. The continuum results suggest that there are no radio-loud AGNs in the nuclear region of this galaxy. The OH 18 cm megamaser emission in IRAS 17208-0014 is detected at various angular resolutions. It has an extent of 170 pc; 110 pc and is mostly localized in two regions separated by 61 pc. The structure and dynamics of the maser emission seem to be consistent with a clumpy, rotating, ringlike geometry, with the two dominant maser regions marking the tangential points of the proposed rotating-ring distribution. Assuming Keplerian motion for the rotating maser ring, the enclosed dynamical mass and the mass density within a radius of 30.5 pc are about 3 x 10(7)(sin(-2) i) M circle dot and 281(sin(-2) i) M circle dot pc(-3), respectively.

We present sensitive VLBI (VLBA + Phased VLA) results on the radio continuum of, and the 21 cm H I absorption toward, the ULIRG IRAS 17208-0014 at 1382 MHz. The high-resolution continuum images reveal a nuclear starburst region in this galaxy. This nuclear region is composed of diffuse emission that extends over 670 x 340 pc on the plane of the sky, and a number of compact sources. The compact sources are likely to be clustered supernova remnants and/or luminous radio supernovae. We derive a massive star formation rate of similar to 84 M-circle dot yr(-1), and a supernova rate of similar to 4 yr(-1). The HI absorption line associated with this galaxy is very wide (Delta V-20% = 696 km s(-1)). It is composed of multiple components with optical depths between 0.3 and 2.5. The H I absorption shows a strong velocity gradient of 453 km s(-1) across 0."36. Assuming Keplerian motion, the enclosed dynamical mass is about 2.3 x 10(9) (sin(-2)i) M-circle dot, comparable to the mass estimated from CO observations.

Magnetic fields are known to play an important role in star formation. The Zeeman effect remains the most direct method for measuring magnetic field strengths in the interstellar medium. Observations of the Zeeman effect in water masers with the VLBA offer a unique window into magnetic fields in the highest density regions at extreme spatial resolution. We report on a long-term program to measure magnetic field strengths in star forming regions with the VLBA. A highly successful pilot study of circular polarization in water masers yielded field strengths ranging from 14 to 42 milligauss in the W3 IRS 5 star forming region. Previous VLA observations revealed magnetic fields between 20 and 40 milligauss in the W3(OH) and OH43.8-0.1 regions. Clearly, the VLA did not resolve all the maser spots. Higher resolution VLBA observations have been made in order to examine these regions in more detail.

We present phase-referenced VLBI observations of the radio continuum emission from, and the neutral hydrogen 21 cm absorption toward, the luminous infrared galaxy NGC 7674. The observations were carried out at 1380 MHz using the VLBA, the phased VLA, and the 305 m Arecibo radio telescope. These observations constitute the first scientific use of the Arecibo telescope in a VLBI observation with the VLBA. The high- and low-resolution radio continuum images reveal several new continuum structures in the nuclear region of this galaxy. At ~100 mas resolution, we distinguish six continuum structures extending over 1.″4 (742 pc), with a total flux density of 138 mJy. Only three of these structures were known previously. All these structures seem to be related to active galactic nucleus (AGN) activity. The overall S-shaped pattern that the radio structures seem to form could be the result of the interstellar medium diverting the outcoming jets from the central AGN. However, we cannot rule out the possibility of a black hole merger that could result in a similar structural pattern. At the full resolution of the array (11 × 5 mas), we detect only two of the six continuum structures. Both are composed of several compact components with brightness temperatures on the order of 107 K. While it is possible that one of these compact structures could host an AGN, they could also be shocklike features formed by the interaction of the jet with compact interstellar clouds in the nuclear region of this galaxy. Complex H I absorption is detected with our VLBI array at both high and low angular resolution. Assuming that the widest H I feature is associated with a rotating H I disk or torus feeding a central AGN, we estimate an enclosed dynamical mass of ~7 × 107 M☉, comparable to the value derived from the hidden broad Hβ emission in this galaxy. The narrower H I lines could represent clumpy neutral hydrogen structures in the H I torus. The detection of H I absorption toward some of the continuum components and its absence toward others suggest an inclined H I disk or torus in the central region of NGC 7674. The overall averaged H I spectrum toward the continuum structures with H I absorption is very consistent with the Arecibo single-dish H I absorption spectrum at 3.′3 resolution.