We have discovered maser emission from SiO and H2O molecules toward a number of evolved stars within the central parsec of our Galaxy. The maser positions can be registered with milliarcsecond precision relative to the radio continuum emission of the nonthermal Galactic center source Sgr A*. Since the masing stars are prominent infrared sources, our data can be used to locate the position of Sgr A* on infrared images of the Galactic center region. Using VLBA observations it will be possible to measure proper motions of the maser stars, which can be used to put constraints on the mass distribution in the central parsec.

We present evidence for a compact ( ≈ 50pc scale) rotating H ɪ disk or torus in Cygnus A, which may represent part of the structure which hides the quasar nucleus.

We have carried out VLBA observations of the OH absorption lines toward the nucleus of the active galaxy, NGC 5793, which is known to have a megamaser in its center. We have also successfully made its continuum image at the L band. It reveals the typical “core-jet” structure on parsec scales. Uniform values of optical depth at each point against the background radio continuum source shows that the column densities of the OH absorbing cloud uniformly distributes around the radio compact nucleus.

The relative proper motions of water vapor maser features in the H ɪɪ region IC 133 have been used to determine a distance to the galaxy M33 that is independent of the usual calibrations associated with extragalactic distance estimates, e.g., extinction and metallicity.

VLBA observations of water-vapor maser emission in the active nucleus of a Seyfert 2, IC 2560, show linear velocity gradient along the north-south elongation, suggesting a compact rotating disk. The binding mass density within the disk is 7.7 × 106 M⊙ pc−3. We are also monitoring the velocity variations of the maser features with single dish telescopes. By combining the velocity drift with the linear velocity gradient, we would be able to determine the rotation speed and radius of the nuclear disk, and hence the central mass.

We have imaged the H2O maser and 22, 8, & 5 GHz continuum in the nucleus of NGC 3079, using the NRAO VLBA. The maser features are distributed over ~ 2 pc along an axis aligned with the plane of the kpc–scale edge–on molecular disk. The masers are not angularly coincident with any detected continuum emission. The two brightest continuum features, which trace a parsec–scale jet, have similar spectra that peak at frequencies ν > 5 GHz. We also detected faint maser emission along the jet axis.

We report on the first results of our ground-survey of H2O masers to establish a list of suitable candidates for space-VLBI observations. Due to the large VSOP/HALCA baselines and its limited sensitivity only very strong and compact sources will be detected. The best candidates from this survey will be part of The VSOP Survey which intends to observe a large number of continuum and maser sources. This systematic and uniform survey, of a large number of sources, will be invaluable in determining the properties of the sub-milliarcsecond emission from maser sources.

We have observed ν = 1, J = 1 – 0, 43-GHz SiO maser emission towards the symbiotic Mira variable R Aqr at four different epochs in the stellar pulsation cycle using the VLBA.

We report on 4 epochs of VLBA polarization observations of the 43 GHz, v=1, J=1–0 SiO masers in the circumstellar envelope around the Mira variable U Herculis.

We measured relative positions of the 22 GHz H2O maser components toward the class 0 protostar S106FIR with the VLA. Two clusters of H2O masers were found. The separation between these clusters is approximately 50 AU at position angle = 73° and each cluster is spread over about 10–20 AU The size of the S106FIR maser clusters is unusually compact compared with other known H2O masers associated with outflows of forming stars. The maser emission appears to originate from MHD driven outflow very close to the star.

We compare spatial structure of the H20 maser during an active period with that during a later quiescent period, received with VLBA in 1993.

Magnetic field parameters and other characteristics of the postshocked gas in W 49 N are derived from a linewidth comparison between the multi-epoch VLBI data of Gwinn and contemporaneous long-term single-dish observations. The observations support predictions of dissociative shock models.

We present measurements of proper motions for circumstellar water maser observations obtained with the VLBA. Results of the observations and modeling indicate that the water masers exist in a kinematically complex region of the circumstellar envelope. A change in position of the maser spots as a function of velocity is discussed.

Eight Class I methanol masers were imaged at 44 GHz with A–configuration of the VLA providing angular resolution 0″.2. The images show a distribution of unresolved maser spots forming pairs in simple structure sources, and a distribution of pair clusters in complex sources. We found that the maser pairs are associated with hot, dense gas cores of sub–stellar masses. In complex sources the cores were found over the whole extent of the host molecular clouds.

The characteristics of rapid variability of flat-spectrum radio sources are reviewed. A large fraction of the blazar population is found to show variability on timescales shorter than one day throughout the entire electromagnetic spectrum. The spectral indices and polarization characteristics change equally fast.

Structure functions of the well-monitored sources show pronounced breaks on scales of about 10 to 50 hours, with flatter slopes towards the fast end. This illustrates that Intraday Variability (IDV), i.e. the high frequency end of the power spectrum is qualitatively different and requires different mechanisms than slower variations.

While intrinsic IDV provides direct clues on small-scale structure over fifteen decades in frequency, extrinsic contributions from interstellar scattering contributes at the lowest frequencies, and remains difficult to disentangle from the intrinsic effects.

Integrated VLA measurements during our 6 cm global VLBI polarization observations of the intraday variable sources 0716+714, 0917+624, and 0954+658 indicate that although there were no significant total intensity variations during the 24-hour VLBI experiment, the polarization for all three sources varied substantially. Independent polarization images made for 2–3 hour segments show that there were no variations on VLBI scales for 0716+714 or 0954+658, but that there were appreciable variations in the VLBI core of 0917+624, which correlate well with the integrated variations in the simultaneous VLA data.

We have found large amplitude intraday variability in the radio quasar PKS 0405–385 on timescales less than an hour. If intrinsic to the source, the inferred brightness temperature is TB ~ 1021 K, far exceeding the inverse Compton limit for a static synchrotron source. We argue that our data are in agreement with interstellar scintillations of a source component which is < 5μarcsec in size.

Sources that are compact enough to show intrinsic variability on times of a day or less (IDV) at cm wavelengths must also show interstellar scintillation (ISS) on similar timescales. However for many IDV sources, the variations could be entirely due to ISS, reducing the implied brightness temperatures to ~ 1013 K or less.

This is a progress report on the ATCA IDV survey of compact, flat or inverted spectrum radio sources. We found that four sources: PKS 0405–385, PKS 1034–293, PKS 1144–397, and PKS 1519–273 out of the sample of 125 show high flux density variability on the daily timescale. The characteristics of observed IDV are discussed and we reflect on its possible origin.

Global λ = 6 cm VLBI polarization observations of intraday variability in the three BLLacertae objects 1334–127, 2131–021, and 2155–152 are analyzed. Integrated VLA measurements during the VLBI observations show that although there were no substantial total intensity variations, the polarizations for these three sources varied significantly during the VLBI experiment. The VLBI data were divided into 2-3 hour segments in order to search for corresponding rapid variability in the VLBI polarization structure. Our analysis shows that the VLA polarization variability in 2155–152 and 2131–021 is present in the VLBI data (i.e., it occurred on mas scales), but this is not the case for 1334–127.