We have used the Very Long Baseline Array (VLBA) and the Very Large Array (VLA) to perform multi-wavelength radio imaging of the RS CVn active binary UX Arietis during the period November 16–21 1995. A series of strong flaring events exhibiting optically-thick spectra peaking above 100 GHz and extended source structures consistent with inter-binary emission were detected.

We report first results from a multiwavelength campaign to measure the simultaneous spectrum of Sgr A* from cm to mm wavelengths. The observations confirm that the previously detected submm-excess is not due to variability; the presence of an ultracompact component with a size of a few Schwarzschild radii is inferred. In a VLA survey of LINER galaxies, we found Sgr A*-like nuclei in one quarter of the galaxies searched, suggesting a link between those low-power AGN and the Galactic Center.

From the combination of VLBI phase-referenced observations and Hipparcos satellite data, we have found evidence of a low-mass object orbiting the late-type star AB Doradus. The mass of the new object is near the hydrogen burning limit and will constitute a precise point for calibrating the low end of the main sequence. This represents the first detection of a low-mass stellar companion using the VLBI technique, which could become an important tool in future searches for planets and brown dwarfs orbiting other stars.

Radio-wave scattering in the Vela supernova remnant acts as an imperfect lens to resolve the pulsar’s radio emission region. We use this lens to measure the pulsar’s emission region. We suggest that refraction of radiation within the pulsar’s magnetosphere is responsible for the observed size.

Interstellar scattering dictates the limiting resolution for many pulsars and OH masers and some extragalactic sources, a limit made more important by the advent of spacebased VLBI. Scattering observations have been biased toward the inner Galaxy, leaving largely unconstrained basic parameters such as the scattering medium’s extent in the outer Galaxy. Ionized gas at ~ 50 kpc is suggested by the appearance of H ɪ in nearby galaxies and models of low-redshift quasar absorption systems. We combine multiwavelength, VLBA observations of twelve extragalactic sources with previous scattering measurements of extragalactic sources and pulsars in the anticenter and constrain the radial extent of the scattering medium to 20 kpc, comparable to that of massive star formation. The H ɪɪ disk does not display significant flaring or warping, though this conclusion may reflect the coarse sampling.

We present the VLBI determined position of the millisecond pulsar PSR B1257+12 which has been shown to have a planetary system (Wolszczan & Frail 1992). The position determination for epoch June 20, 1992 is: αJ2000=13h0m3s.05005(5),δJ2000=+12°40’56″.7043(34). We indicate how the combination of such pulsar observations along with timing observations can be used to directly tie the solar-system dynamic reference frames and the extragalactic reference frame.

Monitoring with the Ryle Telescope at 15 GHz of the Galactic X-ray transient source GRS 1915+105 has revealed a remarkable range of rapid and extended flares which appear to be related to the X-ray emission as recorded by the RXTE all-sky monitor. Quasiperiodic oscillations in the range 20–40 min have been found and are probably related to oscillations in the soft X-ray flux.

We initiated a project with the VLBA to measure directly the distance to the Galactic Center, R0, via a trigonometric parallax. Here we describe the observing program and the first results–direct imaging of the effects of the Sun’s orbit about the Galactic Center.

Weak and strong flaring has been observed at cm-wavelengths on Cygnus X-3. The weak flares seem to have different statistical properties to those of the strong flares. Previous observations have shown the ejection of radio emitting blobs at around 0.3c following major flares. However recent VLBA observations show apparent superluminal expansion and contraction during weak flaring.

From three VLBA observations at 5 GHz, spanning 13 months, we have measured the trigonometric parallax of Sco X-1 of 0.23 ± 0.28 mas; hence its distance is > 1300 pc. This supports the hypothesis that the the x-ray luminosity is near the Eddington Limit.

All three VLBA observations show a radio core of flux density 0.5 mJy and size < 4 mas. However, the third VLBA observation revealed two additional radio components, separated by 12 mas (≈ 20 au) on opposite sides of the radio core. The evolution of these new components is unknown until additional observations can be made. The relative luminosity and separation of the two radio components are inconsistent with the Doppler beaming of two identical ejecta from the radio core.

During quasi-periodic flux variations, the core of this galactic superluminal source shows: (a) a flat radio spectrum between 13cm and 2cm; (b) elongation of the core along the axis of arcsecond-scale ejecta; (c) a time-delay of ~4 mins at 3.6cm, relative to 2cm; (d) progressively less variation of the flux at 2cm, 3.6cm & 13cm; and (e) scatter-broadening to 1.9mas at 8.4GHz (135mas at 1GHz).

New radio observations from the Australia Telescope Compact Array of the radio jet X-ray binary Circinus X-1 are summarized. Radio flux density monitoring around nearly an entire orbit is presented, confirming continued radio flaring around phase 0.1 (near periastron passage) as well as a lesser enhancement half an orbit later.

The young star forming region, Cepheus A, has been monitored since 1981, and shows four new types of time-dependent events. Two of these are briefly described.

We report observations of the radio periodic Be X-ray binary LSI+61°303, obtained with the VLA at 3.6 and 2.0 cm during a full 26.5 d cycle. These observations were carried out in coordination with CGRO monitoring of its possible γ-ray counterpart 2CG 135+1.

GX 339–4 has been monitored over a period of two and a half years with MOST and ATCA to investigate the flux variations and spatial structure of its radio emission. The data are summarized and discussed.

A VLBA image of the X-ray binary Cyg X-3 made two days after a 10 Jy radio flare shows a one-sided, curved jet with a size scale of ≈60 mas. This seems to indicate a jet with a velocity of ≥0.89c with a inclination of a few degrees to the line of site, and indicates that some of events where Cyg X-3 flares above 3 Jy may come from highly relativistic jets close to the line of sight.

With new 6 cm observations we confirm the self-similar expansion of SN 1993J previously discovered at 3.6 cm and estimate the expansion deceleration parameter. The results are inconsistent with the existence of a constant pre-explosion stellar wind but otherwise confirm the standard radio supernova model. The first map at 13 cm showing shell structure is also presented.

We report on phase-referenced 5 and 8.4 GHz VLBI images of supernova SN 1993J in M81, tracking the evolution of the source from 50 to 1000 days after the explosion.

The radio emission from supernovae (SNe) is nonthermal synchrotron radiation of high brightness temperature, with a “turn-on” delay at longer wavelengths, power-law decline after maximum with index β, and spectral index α asymptotically decreasing with time to a final, optically thin value. Radio supernovae (RSNe) are best described by the Chevalier (1982) “mini-shell” model, with modifications by Weiler et al. (1990). RSNe observations provide a valuable probe of the SN circumstellar environment and constraints on progenitor masses. We present a progress report on a number of recent RSNe, as well as on new behavior from RSNe 1979C and 1980K, and on RSNe as potential distance indicators. In particular, we present updated radio light curves for SN 1993J in M81.

The radio remnant of SN 1987A is now of sufficient flux density that super-resolution is possible at 9 GHz. We present a sequence of images from 1992 to 1996 with a resolution of ≈ 0″.5xb, showing the continuing interaction between the expanding shock front and the surrounding axisymmetric circumstellar material.