Engineering topics

Preston, R. A.

Publications and source records attributed to Preston, R. A..

At least 127 records · Page 7

NASA NTRS1985

Quasar 4C39.25 is not contracting

A map has been made of the quasar 4C39.25, which has been described as consisting of two components whose angular separation remains constant at about 2 marc-sec, using VLBI observations made at 3.6 cm during 1983. From the map it is concluded that Shaffer's (1984) suggestion that this source may have been contracting superluminally during 1979-82 is not correct. Three distinct components are found in the quasar structure, two separated by 2.0 marc-sec and a third, presumably new and not previously reported, situated between the other two. It is possible either that the third component is stationary and that its flux density has rapidly increased to render it visible, or that it has recently been ejected from the westernmost component.

Marcaide, J. M.↗

NASA NTRS1985

Arcsecond positions for milliarcsecond VLBI nuclei of extragalactic radio sources. III - 74 sources

VLBI measurements at 2290 and 8420 MHz on baselines of 10,000 km have been used to determine the positions of the milliarcsecond nuclei in 74 extragalactic radio sources. Estimated accuracies range from 0.1 to 4.3 arcsec in both right ascension and declination with typical accuracies of about 0.3 arcsec. The observed sources are part of an all-sky VLBI catalog of milliarcsecond radio sources. Arcsecond positions have been determined for 819 sources. These positions are presently being used to identify optical counterparts in the southern hemisphere.

Morabito, D. D.↗

NASA NTRS1985

Arcsecond Positions for Milliarcsecond VLBI Nuclei of Extragalactic Radio Sources. Part 3: 74 Sources

VLBI measurements at 2290 MHz and 8420 MHz on baselines of 10,000 km between Deep Space Network stations have been used to determine the positions of the milliarcsecond nuclei in 74 extragalactic radio sources. Estimated accuracies range from 0.1 sec. to 4, 3 sec. in both right ascension and declination with typical accuracies of approx. 0.3 sec. The observed sources are part of an all-sky VLBI catalog of milliarcsecond radio sources. Arcsecond positions have now been determined for 819 sources. These positions are presently being used to identify optical counterparts in the Southern Hemisphere.

Morabito, D. D.↗

NASA NTRS1985

The Venus Balloon Project

During the Soviet Vega Mission to Venus and Comet Halley, two instrumented balloons will be placed into the Venusian atmosphere in June 1985. These Soviet/French balloons will be used to study the structure and dynamics of the Venusian atmosphere by means of in situ measurements and earth-based VLBI determination of balloon position and velocity. The DSN 64-meter subnet will be part of an international network of antennas organized by the French to support this mission. The DSN is installing new L-band receiving systems for this task. All scientific data from the balloons will be analyzed by a joint Soviet/French/U.S. science team.

Preston, R. A.↗

NASA NTRS1985

Dual polarization VLBI observations of stellar binary systems at 5 GHz

The binary systems Sigma CrB, SZ Psc, HR 1099, UX Ari, HR 5110, Algol, and Cyg X-1 are characterized in terms of marcsec-scale radio structure on the basis of dual-polarization observations at 4.969-4.997 GHz, obtained with an international VLBI array comprising the 100-m Effelsberg, 36-m Haystack, 42-m Green Bank, 26-m Fort Davis, 130-m-equivalent VLA, and 40-m Owens Valley telescopes (system temperatures 40-70 K) at 22-14 h UT on July 26-27, 1983. The data are presented in graphs and tables and discussed in detail. Single Gaussian brightness distributions with theta(FWHM) = 0.5-2.0 marcsec and brightness temperatures (2-200) x 10 to the 8th K are determined for four sources; core-halo structures are found for UX Ari and Algol; and an upper limit of 50 microarcsec is established for the angular separation of the regions of left and right circular polarization. The gross features seen in RS CVn systems are explained in terms of a simple expanding-coronal-loop model of flare events.

Mutel, R. L.↗

NASA NTRS1985

The quasars 1038 + 528 A and B

The results of VLBI observations of the quasars 1038 + 528 A and B at 2.8, 3.6, 13, and 18 cm at various times between November 1979 and March 1981 are reported. The observations and data calibration are described, as are the mapping and astrometric techniques applied in the study. Both quasars are found to have 'core-jet' morphologies. The core of the A quasar dominates its morphology at centrimetric wavelengths with the brightness temperature of its 400 pc long jet being about 1/100 that of the core. By contrast, the 'jet' in the B quasar is very short (about 70 pc); the tail of this jet has the steepest spectral index found to date in extragalactic compact sources, indicating that high electron losses are responsible for the shortness of the jet. No evidence for appreciable morphological change in the B quasar was found over the time span of the study, whereas a new feature may be emerging from the A quasar core at superluminal speed.

Marcaide, J. M.↗

NASA NTRS1985

Simultaneous dual-wavelength VLBI observations of the compact radio source near the Galactic center

Preliminary results from simultaneous VLBI observations at 3.6 and 13 cm confirm that, in this wavelength range, the size of the compact radio source near the Galactic center varies as lambda-squared, where lambda is the wavelength of observation. These results also establish a flux density upper limit at 3.6 cm of 5 mJy for any source in Sgr A West smaller than 8 mas (80 AU).

Marcaide, J. M.↗

NASA NTRS1985

High-angular resolution observations of stellar binary systems

A long-term program designed to investigate the spatial structure of centimetric radio emission from close binary systems using multistation VLBI array is discussed. Eleven binaries, including eight RS CVn systems, Algol, LSI 61 deg 303, and Cyg X-1, have been detected. The measured brightness temperatures vary from T(B) about 10 exp 8.5 K during periods of low activity to T(B) about 10 exp 10.5 K during flares. Extensive observation of a few sources has shown that the spatial structure is 'core-halo' with linear dimensions of about a stellar radius and the binary system, respectively. The observations are consistent with gyrosynchrotron emission of mildly relativistic electrons in magnetic fields of about 10 exp 1.5 + or - 0.5 gauss. The core sources appear to be optically thick, while the halo component is optically thin.

Lestrade, J.-F.↗

NASA NTRS1984

The milli-arcsecond images of Q0957 + 561

VLBI observations at 13 cm have detected similar compact radio sources in the A and B images of the quasar QO957 + 561, and the images are noted to contain core components with flux densities of 22 + or - 1 and 18 + or - 1 mJy, respectively. Data from the short Effelsberg-Madrid baseline suggest that additional correlated flux density, distributed over an angular scale of about 20 milliarcsec accompanies both cores. The observed brightness distributions of these cores are related by a linear magnification matrix, consistent with observation of surface brightness, as would be expected for images produced by a transparent gravitational lens. A search was conducted for evidence of additional compact radio structure within a region encompassing the positions of the galaxy G1 and the B image.

Gorenstein, M. V.↗

NASA NTRS1984

A search at the millijansky level for milli-arcsecond cores in a complete sample of radio galaxies

A complete sample of 26 extended radio galaxies was observed at 2.29 GHz with the Mark III VLBI system. The fringe spacing was about 3 milli-arcsec, and the detection limit was about 2 millijanskys. Half of the galaxies were found to possess milli-arcsec radio cores. In all but three sources, the nuclear flux density was less than 0.04 of the total flux density. Galaxies with high optical luminosity (less than -21.2) were more likely than less luminous galaxies to contain a detectable milliparcsec radio core (69 percent vs. 20 percent). For objects with arcsec cores, 80 percent were found to have a milli-arcsec core, even though the milli-arcsec object did not always contribute the greater part of the arcsec flux density.

Wehrle, A. E.↗

NASA NTRS1984

A study on the imaging potential of QUASAT

The orbit, ground array, and sensitivities of the space and ground systems of the QUASAT spaceborne astronomy mission were studied. Parameters which yield images in which the noise is within a factor of three of the thermal noise limit were derived. With a very large ground array QUASAT should produce images as complex as the best of the VLA. At 22 GHz the resolution will be 0.1 milliarcsec, the field of view 15 milliarcsec, and it will be possible to make noise limited maps of complex objects. The QUASAT is essential for mapping southern declination objects. Without QUASAT, even the simple core-jet source cannot be mapped at declination minus 60 deg. It will be possible with QUASAT to make maps with observing times as short as 6 hr although the very best maps will require 48 hr observations.

Readhead, A. C. S.↗

NASA NTRS1984

The radio and optical identification of Circinus X-1

VLBI and CCD astrometry is presented which establishes the coincidence of the X-ray and radio object Cir X-1 with an object lying about one arcsec south of the object 'L' of Whelan et al. (1977), a conclusion reached previously by Argue and Sullivan (1982). The question whether the object L has any physical connection with Cir X-1 cannot yet be definitely answered.

Argue, A. N.↗

NASA NTRS1984

Detection of a sub-milliarcsecond radio component in the RS CVn system HR 1099

The RS CVn system HR 1099 was observed with a five station VLBI array at a frequency of 8.4 GHz during a strong radio outburst of approximately 400 mJy. The data are consistent with a circular Gaussian source of 0.8 + or - 0.12 milli-arcsec (FWHM), corresponding to a linear size of 4 + or - 0.6 x 10 to the 11th cm. This is comparable to the distance between the surfaces of the two stars, or to 75 percent of the diameter of the chromospherically active K star. Extrapolation of published photometric data shows that the starspot formation of HR 1099 was facing toward the observers at the times of observations. The high equivalent brightness temperature, approximately 10 billion K, is consistent with gyrosynchrotron emission from a power-law energy distribution of electrons in a magnetic field of strength about 30 gauss.

Mutel, R. L.↗

NASA NTRS1984

Very long baseline interferometry observations of the RS Canum Venaticorum system HR 5110 at 8.4 GHz

The RS CVn binary system HR 5110 was observed with a well-calibrated four-element VLBI array at a frequency of 8.4 GHz. The total flux density, which was nearly constant at 32 + or - 2 mJy, was monitored throughout the experiment with a 20 km baseline interferometer allowing accurate visibilities to be calculated. The observed visibilities are consistent with a source unresolved on all the baselines. The maximum size of the equivalent Gaussian source is 1.4 milli-arcsec (FWHM), corresponding to a linear size of 1.1 x 10 to the 12th cm at 52 pc. This maximum size is comparable with the overall size of the binary system. The inferred lower limit of the brightness temperature is 4 x 10 to the 8th K, which is higher than the coronal temperature of 10 to the 7th K measured in a soft X-ray survey. This is consistent with a nonthermal mechanism for the radiation process of the moderate radio outburst observed.

Mutel, R. L.↗

NASA NTRS1984

Arcsecond Positions for Milliarcsecond VLBI Nuclei of Extragalactic Radio Sources, Part 2: 207 Sources

Very long base interferometry measurements of time delay and fringe frequency at 2.29 GHz on baselines of 10,000 km between Deep Space Network stations were used to determine the positions of the milliarcsecond nuclei in 207 extragalactic radio sources. Estimated accuracies generally range from approximately 0.1 to approximately 1.0, in both right ascension and declination, with all sources having uncertainties 4" in both coordinates. The observed sources are part of an all-sky VLBI catalog of milliarcsecond radio sources. Arcsecond positions are now determined for 752 of these sources. Arcsecond positions serve as a useful starting point in the construction of high-precision VLBI reference frames and are also important for unambiguous determination of optical counterparts to compact radio sources.

Morabito, D. D.↗

NASA NTRS1984

Very Long Baseline Interferometry observations of 257 extragalactic radio sources in the ecliptic region

Very Long Baseline Interferometry observations of 257 extragalactic radio sources with 10 deg of the ecliptic have been conducted at a frequency of 2.29 GHz. Compact components with flux densities greater than 0.1 Jy and angular sizes smaller than the fringe spacings of 2.5 and 3.2 milliarsec were detected in 144 radio sources. This survey was conducted to find compact radio sources to form a high accuracy reference frame for planetary spacecraft navigation. This stable reference frame may also be useful for long-term studies of planetary dynamics.

Wehrle, A. E.↗

NASA NTRS1984

Radio interferometry from space platforms

While current VLBI observations are limited in their resolution by the earth diameter magnitude, which is the largest antenna separation available, as well as in their information content, because of the small number of antennas in use at a given time, the extension of VLBI to include one or more antennas in space will relieve both constraints and help to map distant radio sources with the highest possible resolution. Attention is given to the implementation of such an orbital VLBI system extension through (1) a Shuttle-launch mission, (2) a six-month to one-year near earth orbit mission based on a space platform associated with the Space Station, (3) a large orbit free flyer platform mission of more than 2-year duration, and (4) lunar and/or deep space orbits, aimed at reaching the resolution limits set by interstellar scattering.

Roberts, D. H.↗

NASA NTRS1983

A Southern Hemisphere VLBI Survey on a 275-km Baseline

A very long base interferometry (VLBI) survey at 2.29 GHz was conducted using a 275 km baseline consisting of the NASA Deep Space Network tracking site in Tidbinbilla, Australia, and the 64 m antenna located at Parkes, Australia. The purpose of the survey was to identify sources in the southern sky possessing strong compact cores ( 0.1 arcseconds). Such sources will be used to form a reference frame for conducting VLBI geodesy experiments in the Southern Hemisphere. The 70 candidate sources were chosen to be south of -39 degrees declination, and only four had been previously observed from the northern hemisphere. Of the observed sources, 49 were found to have compact structure. In addition to determining correlated flux densities, the delay and delay rate observables of several detected sources were used to determine an estimate of the three dimensional location of the Parkes antenna relative to the Tidbinbilla site with a 1-sigma accuracy of 10 meters.

Morabito, D. D.↗