Bernard F. Burke

Very Long Baseline Interferometric Observations Made with an Orbiting Radio Telescope

An orbiting spacecraft and ground observatories have been used to obtain interferometric observations of cosmic radio sources. The Tracking and Data Relay Satellite System (TDRSS) was used as the orbiting observatory in conjunction with two 64- meter radio telescopes at ground observatories, one in Australia and one in Japan. The quasars 1730-130 (NRAO 530), 1510-089, and 1741-038 were observed at a frequency of 2.3 gigahertz, and a maximum projected baseline of 1.4 earth diameters was achieved. All quasar observations for which valid data were acquired resulted in detected fringes. Many of the techniques proposed for a dedicated very long baseline interferometry observatory in space were used successfully in this experiment.

Discovery of a New Gravitational Lens System

A new gravitational lens system, the triple radio source MG2016+112, has been discovered. Five emission lines at a redshift of 3.2733�0.0014 have been identified in the spectra of two stellar objects of magnitude 22.5 coincident with radio components 3.4 arc seconds apart. The lines are the narrowest ever observed in objects at such a large redshift. The redshift of a 23rd-magnitude extended optical object coincident with the third radio component has not been determined spectroscopically, but its known optical properties are consistent with those of a giant elliptical galaxy with a redshift of about 0.8.

The radio time delay in the double quasar 0957 + 561

Gravitational lens models for the double quasar 0957+561 predict that there should be a time delay of roughly 1 year between the arrival of wave fronts from the two images. In order to measure this delay, the flux densities were monotored at λ = 6 cm of 0957+561A and 0957+561B since 1979. The flux curves of the two images constructed from the VLA data are presented, which are accurate to 2% of the image flux densities. The VLA data are best fitted by a time delay of 1.40 ± 0.1 yr (513 ± 40 days, A leading), with a magnification ratio (B/A) of 0.697 ± 0.003. The error estimates were obtained through Mont Carlo methods, and include 68% of the simulated solutions

The MIT-Green Bank (MG) 5 GHz survey

The catalog yielded by the MIT-Green Bank 5-GHz survey contains 5974 sources with S/N ratio greater than 5; a supplemental catalog contains 3836 possible detections with S/N of less than 5. The reliability of the main list is measured to be about 96 percent, with a completeness of about 95 percent. Flux density errors are a function of flux density. A comparison is presently made of survey sources from the Douglas et al. (1980) 365-MHz Texas survey; spectral indices are computed for coincident sources, and the distribution of spectral indices is discussed.

Magnetic field structure of the star-forming region W3(OH) - VLBI spectral line results

VLBI observations of the ground state 1665 MHz OH emission source in W3(OH) are presented, and 81 maser components stronger than 1.5 Jy have been detected. The observations have also led to the detection of five Zeeman pairs. The percent of linear, circular, and total polarization of the emission of each feature has been determined. Information is provided on the three-dimensional structure of the magnetic field. The results suggest that the OH cloud is permeated with a magnetic field, pointing away from the earth, having an average strength of 6 mG and with a SE-NW projected orientation on the plane of the sky. 29 references.

Precision Geodesy via Radio Interferometry

Very-long-baseline interferometry experiments, involving observations of extragalactic radio sources, were performed in 1969 to determine the vector separations between antenna sites in Massachusetts and West Virginia. The 845.130-kilometer baseline was estimated from two separate experiments. The results agreed with each other to within 2 meters in all three components and with a special geodetic survey to within 2 meters in length; the differences in baseline direction as determined by the survey and by interferometry corresponded to discrepancies of about 5 meters. The experiments also yielded positions for nine extragalactic radio sources, most to within 1 arc second, and allowed the hydrogen maser clocks at the two sites to be synchronized a posteriori with an uncertainty of only a few nanoseconds.

The Radio Wavelength Time Delay of Gravitational Lens 0957+561

The gravitational lens 0957+561 was monitored with the Very Large Array from 1979 to 1997. The 6 cm light-curve data from 1995 to 1997 and the 4 cm data from 1990 to 1997 are reported here. At 4 cm the intrinsic source variations occur earlier and are twice as large as the corresponding variations at 6 cm. The VLBI core and jet components have different magnification factors, leading to different flux ratios for the varying and nonvarying portions of the VLA light curves. Using both the Press, Rybicki, & Hewitt Q (PRHQ) and dispersion statistical techniques, we determined the time delay, core flux ratio, and excess nonvarying B image flux density. The fits were performed for the 4 and 6 cm light curves, both individually and jointly, and we used Gaussian Monte Carlo data to estimate 68% statistical confidence levels. The delay estimates from each individual wavelength were inconsistent given the formal uncertainties, suggesting that there are unmodeled systematic errors in the analysis. We roughly estimate the systematic uncertainty in the joint result from the difference between the 6 and 4 cm results, giving 409±30 days for the PRHQ statistic and 397±20 days for the dispersion statistic. These results are consistent with the current optical time delay of 417±3 days, reconciling the long-standing difference between the optical and radio light curves and between different statistical analyses. The unmodeled systematic effects may also corrupt light curves for other lenses, and we caution that multiple events at multiple wavelengths may be necessary to determine an accurate delay in any lens system. Now that consensus has been reached regarding the time delay in the 0957+561 system, the most pressing issue remaining for determining H0 is a full understanding of the mass distribution in the lens.

5 GHz radio structure and optical identifications of sources from the MG survey. II. Maps and finding charts

Nearly 1000 sources from the 5 GHz MIT-Green Bank survey have been observed with the VLA. Radio maps are given for 460 resolved sources, finding charts for 390 optical identifications, and spectral indices between 1.4 and 5 GHz for 632 sources. 20 references.

Spectral Line Interferometry with Independent Time Standards at Stations Separated by 845 Kilometers

An upper limit of 0.02 second of arc has been determined for a hydroxyl radical (OH) emission region associated with the radio source W3, with the use of a Michelson interferometer consisting of two radio telescopes 845 kilometers apart. Timing was provided at the stations by independent atomic frequency standards. The 1665-megahertz radiation was translated to video frequency and recorded digitally on magnetic tapes which were later processed by computer, yielding fringe phase and amplitude as a function of frequency over the received bandwidth.

A gravitationally-lensed ring in MG 1549+3047

We report the discovery of a new gravitational lens in the radio source MG 1549+3047. The source has a compact core and two lobes, with a faint optical counterpart to the core. The brighter lobe is shaped like a ring, and centered on a foreground galaxy with known redshift. The radio structure of the ring can be reproduced by distorting a typical radio lobe through a simple elliptical gravitational lensing potential, which models the mass distribution in the foreground galaxy. From a preliminary lens model, we show that the mass distribution follows the observed luminosity, and we predict a central velocity dispersion of about 230 km s −1 . This corresponds to a blue mass-to-light ratio of about 18h solar units