Anthony J. Beasley

THE VLBA CALIBRATOR SURVEY—VCS1

A catalog containing milliarcsecond-accurate positions of 1332 extragalactic radio sources distributed over the northern sky is presented—the Very Long Baseline Array Calibrator Survey (VCS1). The positions have been derived from astrometric analysis of dual-frequency 2.3 and 8.4 GHz VLBA snapshot observations; in a majority of cases, images of the sources are also available. These radio sources are suitable for use in geodetic and astrometric experiments, and as phase-reference calibrators in high-sensitivity astronomical imaging. The VCS1 is the largest high-resolution radio survey ever undertaken and triples the number of sources available to the radio astronomy community for VLBI applications. In addition to the astrometric role, this survey can be used in active galactic nuclei, Galactic, gravitational lens, and cosmological studies. Subject headings: astrometry — radio continuum: general — reference systems — surveys — techniques: interferometric On-line material: machine-readable tables

Time-sequenced multi-radio-frequency observations of Cygnus X-3 in flare

Multifrequency observations from the Very Large Array, Very Long Baseline Array (VLBA), and Owens Valley Radio Observatory Millimeter Array of a major radio outburst of Cygnus X-3 in 2001 September are presented, measuring the evolution of the spectrum of the source over 3 decades in frequency, over a period of 6 days. Following the peak of the flare, as the intensity declines the high-frequency spectrum at frequency ν steepens from ν-0.4 to ν-0.6, after which the spectral index remains at this latter terminal value, a trend previously observed but hitherto not satisfactorily explained. VLBA observations, for the first time, track over several days the expansion of a sequence of knots whose initial diameters are ~8 mas. The light-crossing time within these plasmons is of the same order as the timescale over which the spectrum is observed to evolve. We contend that properly accounting for light-travel time effects in and between plasmons that are initially optically thick but that after expansion become optically thin explains the key features of the spectral evolution, for example, the observed timescale. Using the VLBA images, we have directly measured for the first time the proper motions of individual knots, analysis of which shows a two-sided jet whose axis is precessing. The best-fit jet speed is β ~ 0.63, and the precession period is ~5 days, significantly lower than that fitted for a previous flare. Extrapolation of the positions of the knots measured by the VLBA back to zero separation shows this to occur approximately 2.5 days after the detection of the rise in flux density of Cyg X-3.

A Relativistic Jet in the Radio-Quiet Quasar PG 1407+263

We present the results of a multiepoch radio monitoring campaign measuring the milliarcsecond structure of the jet in the radio-quiet quasar PG 1407+263. This is the highest sensitivity, highest resolution multiyear study of a distant active galaxy. The observations are naturally explained in terms of a beamed relativistic jet, some of whose fluctuations in flux density can be ascribed to interaction with the narrow-line region of the quasar. The optical properties of PG 1407+263, in particular the low equivalent widths of the emission lines, may be related to the fact that we are viewing this quasar almost pole-on, giving us a direct view into its broad-line region.

CARMA: a new heterogeneous millimeter-wave interferometer

A new Combined Array for Research in Millimeter-wave Astronomy (CARMA) interferometer is being assembled from the existing Owens Valley Radio Observatory (OVRO), the Berkeley-Illinois-Maryland Association (BIMA) millimeter interferometers and the new Sunyaev?Zeldovich Array (SZA) at Cedar Flat, a site at 2,200 m altitude in the Inyo Mountains east of OVRO. The array will consist of 23 antennas of three different diameters, 3.5, 6.1 and 10.4 m, and will support observations in the 1 cm, 3 mm and 1.3 mm bands. The fist-light correlator is a flexible FPGA based system that will process up to 8 GHz of bandwidth on the sky for two subarrays consisting of 8 and 15 elements. The array configurations will offer antenna spacings from 5 m to 1.9 km allowing unprecedented high resolution and wide field imaging at millimeter wavelengths. Radiometers observing the 22 GHz water vapor emission line will be used to measure and correct for the water vapor induced path delay along the line of sight for each telescope and thereby minimize the time lost to “bad seeing”. This university based facility will emphasize technology development and student training along with leading edge astronomical research in areas ranging from Sunyaev-Zeldovich effect galaxy cluster surveys to studying protoplanetary disks.

DETECTION OF THE NEUPERT EFFECT IN THE CORONA OF AN RS CANUM VENATICORUM BINARY SYSTEM BY XMM-NEWTON AND THE VERY LARGE ARRAY

The RS CVn-type binary σ Geminorum was observed during a large, long-duration flare simultaneously with XMM-Newton and the Very Large Array. The light curves show a characteristic time dependence that is compatible with the Neupert effect observed in solar flares: the time derivative of the X-ray light curve resembles the radio light curve. This observation can be interpreted in terms of a standard flare scenario in which accelerated coronal electrons reach the chromosphere, where they heat the cool plasma and induce chromospheric evaporation. Such a scenario can hold only if the amount of energy in the fast electrons is sufficient to explain the X-ray radiative losses. We present a plausibility analysis that supports the chromospheric evaporation model.

A Methanol Maser Survey of IRAS -Selected Regions in the Magellanic Clouds

We report the results of an Australia telescope 6.6 GHz methanol maser survey of 55 positions in the Large and Small Magellanic Clouds, selected on the basis of their IRAS colors to be ultracompact H II regions. In addition, 12 regions in the LMC possessing compact Hα and radio continuum emission were observed. A new cluster of strong masers (∼ 1-4 Jy) was detected from one source, IRAS 05011-6815; no other detections are claimed above a typical 7 σ level of ∼700 mJy beam -1 . A comparison of the Galactic and Magellanic Cloud 6.6 GHz methanol maser peak flux density distributions indicates a significantly lower detection rate in the latter. We examine the effects of the lower metal and dust abundances of the Magellanic Clouds on the production of saturated methanol masers, which may explain this deficiency.

The Atacama Large Millimeter/submillimeter Array

The Atacama Large Millimeter/submillimeter Array (ALMA) is an international radio telescope under construction in the Atacama Desert of northern Chile. ALMA will be situated on a high-altitude site at 5000 m elevation, allowing excellent atmospheric transmission over the instrument wavelength range of 0.3 to 3 mm. ALMA will contain an array of up to sixty-four 12-m diameter high-precision antennas arranged in multiple configurations ranging in size from 150 meters up to ~15 km, and a set of four 12-m and twelve 7-m antennas operating in closely packed configurations ~50m in diameter. The instrument will provide both interferometric and total-power astronomical information on high-energy electrons, molecular gas and dust in solar system, our Galaxy, and the nearby and high-redshift universe. In this paper we outline the scientific drivers, technical challenges and construction status of ALMA.

Molecular Absorption in Cen A on VLBI Scales

Centaurus A, the nearest AGN shows molecular absorption in the millimeter and radio regime. By observing the absorption with VLBI, we try to constrain the distribution of the gas, in particular whether it resides in the circumnuclear region. Analysis of VLBA observations in four OH and two H2CO transitions is presented here, as well as molecular excitation models parameterized with distance from the AGN. We conclude that the gas is most likely associated with the tilted molecular ring structure observed before in molecular emission and IR continuum. The formaldehyde absorption shows small-scale absorption which requires a different distribution than the hydroxyl.

Chemical Complexity in Local Diffuse and Translucent Clouds: Ubiquitous Linear C3H and CH3CN, a Detection of HC3N and an Upper Limit on the Abundance of CH2CN

We present Jansky Very Large Array observations of 20 - 37 GHz absorption lines from nearby Galactic diffuse molecular gas seen against four cosmologically-distant compact radio continuum sources. The main new observational results are that \linearC3H\ and \methCN\ are ubiqitous in the local diffuse molecular interstellar medium at \AV\

VLBI Observations of Supernova 1993J: The First 1000 Days

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