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NASA NTRS1988

Supernova 1987A - A radiosphere resolved with VLBI five days after the neutrino burst

The results of VLBI observations of SN1987A are reported. No emission from the supernova above a level of about 20 percent of the supernova's total flux density was detected, although signals were detected from two calibrator sources with amplitudes roughly equal to those determined in earlier VLBI observations. It is inferred that the supernova's radiosphere was resolved, and a lower bound on the radiosphere's radius of 2.2 mas is estimated from an epoch 5.2 days after the neutrino burst. Given the photometric data from the supernova, a distance to the LMC of 50 + or - 5 kpc, and an apparent expansion velocity that varied systematically with time from 18,000-16,000 km/s, as estimated from the blue-shifted H-alpha absorption lines on the days preceding the observations, it is concluded that 5.2 days after the neutrino burst the supernova's radiosphere was at least 2.5 times larger than the inferred blackbody photosphere, and at least as large as the H-alpha line-forming region.

Jauncey, D. L.↗

NASA NTRS1989

Crustal dynamics project data analysis, 1988: VLBI geodetic results, 1979 - 1987

The results obtained by the Goddard VLBI (very long base interferometry) Data Analysis Team from the analysis of 712 Mark 3 VLBI geodetic data sets acquired from fixed and mobile observing sites through the end of 1987 are reported. A large solution, GLB401, was used to obtain earth rotation parameters and site velocities. A second large solution, GLB405, was used to obtain baseline evolutions. Radio source positions were estimated globally while nutation offsets were estimated from each data set. Site positions are tabulated on a yearly basis from 1979 through 1988. The results include 55 sites and 270 baselines.

Ma, C.↗

NASA NTRS1988

VLBI observations of Supernova 1987 A

VLBI observations at 2.3 GHz of SN1987 A on February 28, 1987 yielded no fringes, implying, for an optically thin shell, a lower bound on the (outer) diameter of 1.9 mas. From the comparison of the VLBI and optical results, it is inferred that the radiosphere of SN1987 A was either about equal to, or larger than, the photosphere of the supernova five days after the explosion.

Shapiro, I.↗

NASA NTRS1988

VLBI observations of radio stars

After a discussion of the physicl properties of RS CVn and Algol close binaries, the paper examines VLBI observations of close binaries, results on core-halo in close binaries, and dual-frequency and dual-polarization VLBI observations of Algol. Investigations of X-ray binaries and the exotic object SS433 are considered along with the astrometry of radio stars.

Lestrade, Jean-Francois↗

NASA NTRS1988

VLBI positions of eight stellar systems

Radio positions of eight stellar systems have been derived with accuracies of 3 to 300 milliarcseconds from MkIII VLBI observations conducted with multistation arrays. The best accuracy was obtained during a relatively strong outburst of the RS CVn system HR 5110. The epoch J2000.0 positions are obtained in the Jet Propulsion Laboratory VLBI reference frame of extragalactic radio sources.

Neill, A. E.↗

NASA NTRS1988

VLBI geodesy - 2 parts-per-billion precision in length determinations for transcontinental baselines

VLBI was to make twenty-two independent measurements, between September 1984 and December 1986, of the length of the 3900-km baseline between the Mojave site in California and the Haystack/Westford site in Massachusetts. These experiments differ from the typical geodetic VLBI experiments in that a large fraction of observations is obtained at elevation angles between 4 and 10 deg. Data from these low elevation angles allow the vertical coordinate of site position, and hence the baseline length, to be estimated with greater precision. For the sixteen experiments processed thus far, the weighted root-mean-square scatter of the estimates of the baseline length is 8 mm.

Davis, J. L.↗

NASA NTRS1988

VLBI studies of the nutations of the earth

The analysis of six years of VLBI data has yielded corrections to the coefficients of the seven largest terms in the IAU 1980 nutation series with periods of one year or less, with accuracies approaching the truncation error of this nutation series (0.1 mas). This paper examines the methods used to extract the nutation information from the VLBI data, the calculation of the uncertanties of the resultant corrections to the nutation-series coefficients, and current research on the earth's nutations.

Herring, T. A.↗

NASA NTRS1989

A complete sample of double-lobed radio quasars for VLBI tests of source models - Definition and statistics

A complete, flux-density-limited sample of double-lobed radio quasars is defined, with nuclei bright enough to be mapped with the Mark III VLBI system. It is shown that the statistics of linear size, nuclear strength, and curvature are consistent with the assumption of random source orientations and simple relativistic beaming in the nuclei. However, these statistics are also consistent with the effects of interaction between the beams and the surrounding medium. The distribution of jet velocities in the nuclei, as measured with VLBI, will provide a powerful test of physical theories of extragalactic radio sources.

Hough, D. H.↗

NASA NTRS1990

15 GHz space VLBI observations using an antenna on a TDRSS satellite

A 4.9 m diameter antenna in earth orbit, part of the Tracking and Data Relay Satellite System, was previously used for space VLBI observations at 2.3 GHz. It has now been used successfully with two ground antennas for VLBI measurements at 15 GHz. Although the sensitivity (gain/system temperature) of the orbiting antenna was a factor of 6 poorer at 15 GHz than at 2.3 GHz, 11 out of 22 extragalactic sources were detected on space-ground baselines, up to a maximum baseline of 1.63 earth diameters. The results suggest that the distribution of source visibilities at 15 and 2.3 GHz are similar for these baseline lengths. Model-dependent brightness temperatures of 1-2.5 x 10 to the 12th K were derived for six sources. This suggests that sources with brightness temperatures in this range are as common at 15 GHz as at 2.3 GHz. The coherence on space-ground baselines (this includes effects due to the phase transfer from the ground to the satellite and the reconstruction of the spacecraft orbit) was 0.76 for 340 s integrations.

Linfield, R. P.↗

NASA NTRS1990

A demonstration of nanoradian VLBI tracking for deep space navigation

Differential VLBI measurements of the shift in angular position of P 0201+113 due to the effect of Jupiter's gravitational field demonstrate a nanoradian-level, natural-source tracking capability. The high accuracy was achieved by measuring the VLBI delay and delay rate of the target as well as the delays and rates of 5 or more natural radio sources, forming a local reference frame. This accuracy should also be attainable with spacecraft targets. A set of angular positions were inferred for each of two epochs in March and April of 1988. These positions were then differenced to measure the effect of the Jovian gravitational deflection. The differential measurement is largely insensitive to radio source position and structure errors. An additional analysis of the same data, using another source, P 0202+14, as the target, verified that a null planetary gravitational deflection result could also be obtained for a raypath far from any planet. For both targets, the RMS angular scatter of the differential deflections about their expected behavior was about 1.3 nanoradians.

Treuhaft, R. N.↗

NASA NTRS1990

An accuracy assessment of Magellan Very Long Baseline Interferometry (VLBI)

Very Long Baseline Interferometry (VLBI) measurements of the Magellan spacecraft's angular position and velocity were made during July through September, 1989, during the spacecraft's heliocentric flight to Venus. The purpose of this data acquisition and reduction was to verify this data type for operational use before Magellan is inserted into Venus orbit, in August, 1990. The accuracy of these measurements are shown to be within 20 nanoradians in angular position, and within 5 picoradians/sec in angular velocity. The media effects and their calibrations are quantified; the wet fluctuating troposphere is the dominant source of measurement error for angular velocity. The charged particle effect is completely calibrated with S- and X-Band dual-frequency calibrations. Increasing the accuracy of the Earth platform model parameters, by using VLBI-derived tracking station locations consistent with the planetary ephemeris frame, and by including high frequency Earth tidal terms in the Earth rotation model, add a few nanoradians improvement to the angular position measurements. Angular velocity measurements were insensitive to these Earth platform modelling improvements.

Engelhardt, D. B.↗

NASA NTRS1990

Spacecraft Doppler tracking with a VLBI antenna

Preliminary results are reported from Doppler-shift measurements to the Voyager-2 spacecraft at a distance of 26 AU, obtained using the 32-m VLBI antenna at Medicina (Italy) during July and August 1988. The apparatus comprises the el-az antenna, an S-X-band receiver, a hydrogen maser to generate the reference signal, a Mark III VLBI terminal, and a digital tone extractor capable of isolating a tone of known frequency from a noisy signal and giving its phase and amplitude. A signal transmitted in S-band from the NASA Deep Space Network (DSN) station in Australia and retransmitted coherently in X-band by Voyager, was received 7 h 6 min later at Medicina and at the DSN station in Madrid. Sample data are presented graphically and shown to be of generally high quality; further in-depth analysis is under way.

Comoretto, G.↗

NASA NTRS1991

Comparison of VLBI and SLR geocentric site coordinates

Results are reported from a systematic comparison of the geocentric coordinates determined for 18 pairs of VLBI and satellite laser ranging (SLR) sites. The data and results are presented in tables and briefly characterized. The rms differences in the X, Y, and Z coordinates are found, after a 7-parameter frame adjustment, to be 15, 22, and 22 mm, respectively. The potential usefulness of a combined VLBI-SLR reference frame for spacecraft tracking and similar applications is indicated.

Ray, J. R.↗

NASA NTRS1991

Simulations of space VLBI

The paper concentrates on the results from space VLBI simulations undertaken during the past four years, beginning with a study of the imaging potential of the proposed medium-term mission QUASAT, and continuing with the Japanese VSOP and Soviet Radioastron missions. The purpose of the study is to determine what quality of images can be expected from space VLBI, given realistic data errors and spacecraft orbits yielding incomplete UV coverage. Much of the paper is devoted to the problem of spurious symmetrization, and a method of overcoming this problem by using a set of UV-constraints is presented. Constraints limiting the imaging ability of the VSOP and Radioastron projects are discussed, and it is concluded that the application of constraints in either sky- or UV-plane will make it possible to overcome spurious symmetrization.

Murphy, D. W.↗

NASA NTRS1991

VLBI observations of the nuclear jet in Cygnus A

An image of the 'core' and inner jet of Cygnus A is presented at an angular resolution of 2 mas, obtained from Mark III VLBI observations at 6 cm wavelength with an array of nine antennas. The core is resolved, at least partly due to its being broadened by scattering in the Galactic interstellar medium. The jet has several condensations out to a distance of about 25 mas from the core and has a fairly constant FWHM of about 2.2 mas. The parsec-scale jet points toward the arcsec-scale jet observed with the VLA to within 4 deg. The ratio between the flux density of the jet and that of the counterjet is greater than 15:1. From a comparison with an earlier VLBI image of Cygnus A, an upper limit to the apparent velocity of the brightest knot in the jet is found to be about 2c, where c is the speed of light (for H0 = 60 km/sec Mpc, q0 = 0).

Carilli, Christopher L.↗

NASA NTRS1992

A VLBI search for AGN radio cores in starburst galaxies

Results are presented of Mk III 18-cm VLBI observations of a small sample of IR-bright galaxies: four starburst galaxies which have no evidence of an AGN from the optical spectrum, two Seyfert 2 galaxies, and one previously unclassified Markarian galaxies. In Mrk 297 the compact radio source lies in an obscure clump near the edge of the galaxy. This is either the first known case of a detached nucleus (presumably as a result of disruption during a merger) or a radio supernova of unprecedented luminosity. No VLBI-scale emission was detected in the nucleus of the starburst galaxy Mrk 201 or in Mrk 928a, although the Seyfert 2 nucleus Mrk 928b was detected. Mrk 620 was found to have a milliarcsecond core, but Mrk 520 was not detected. Optical spectroscopy was obtained for five galaxies, including Mrk 520. It is demonstrated that, in luminous starburst galaxies, an active nucleus can effectively be completely hidden from view in the optical by dust. The results also support a physical or causal connection between classical nuclear activity and the starburst phenomenon.

Lonsdale, Colin J.↗

NASA NTRS1991

VLBI phase-referencing for observations of weak radio sources

Phase-referencing is a technique used in VLBI to extend the signal coherence time from a few minutes to a few hours in order to enhance significantly its sensitivity. With this technique, VLBI observations of milliJansky radio sources can be conducted for high-accuracy differential astrometry as well as imaging. We describe the technique in some details and present, as an example, a submilliarcsecond differential astrometric experiment design to identify the star responsible for the weak radio emission in the binary system Algol.

Lestrade, J.-F.↗

NASA NTRS1991

Are the Band Width Synthesis delay and delay rate observables useful for VLBI imaging?

The Band Width Synthesis (BWS) delay and delay rate observables measured by VLBI are used in astrometry to determine positions of extragalactic radio sources with a precision of 0.001 arcsec. At this level of precision, extragalactic radio sources exhibit extended structures that induce significant effects in the measured BWS delays and delay rates. The question whether closure BWS delays and delay rates could supplement the amplitudes and closure phases for VLBI imaging is studied. Preliminary results for the radio source NRAO140 observed in July 1983 are presented.

Charlot, P.↗