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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

VLBI collimation-tower technique for time-delay studies of a large ground-station communications antenna

A need for an accurate but inexpensive method for measuring and evaluating time delays of large ground antennas for VLBI applications motivated the development of the collimation tower technique. Supporting analytical work which was performed primarily to verify time delay measurement results obtained for a large antenna when the transmitter was at a collimation distance of 1/25 of the usual far field criterion is discussed. Comparisons of theoretical and experimental results are also given.

Otoshi, T. Y.↗
NASA NTRS1985

Submilliarcsecond VLBI observations of the close pair GC 1342 + 662 and GC 1342 + 663

Differential VLBI was simultaneously performed on the source pair GC 1342 + 662 and GC 1342 + 663 (4.4-arcminute separation) using S-band on the Goldstone/Madrid baseline. These measurements were acquired on two separate observing sessions: 30 December 1982 and 14 May 1983. The change in separation of GC 1342 + 662 relative to GC 1342 + 663 between the two epochs was 0.03 + or - 0.08 milliarcsecond. The differences of the relative position measurements between epochs of GC 1342 + 662 relative to GC 1342 + 663 were -0.29 + or - 0.05 milliarcsecond in right ascension and 0.14 + or 0.09 milliarcsecond in declination. These measurements demonstrate submilliarcsecond accuracy and repeatability. The discrepancies outside of the formal uncertainties could be attributed to the intrinsic properties of the sources such as structure and to a lesser probability, proper motion. These discrepancies could also be attributed to excursions in UT1-UTC of about four times the quoted BIH uncertainty.

Morabito, D. D.↗
NASA NTRS1985

VLBI observations of the jet in Cygnus A

The luminous radio galaxy Cyg A was observed with a five-station US VLBI array at 5 GHz at epoch 1983.25. The jet can be followed from the core to a distance of 9 milliarcsec (mas). Within the limited dynamic range of these observations, it is one-sided (jet:counterjet ratio greater than 12:1). If this one-sidedness is due to Doppler boosting, then (1) the jet must decelerate to a mildly relativistic velocity on a kiloparsec scale, (2) the source cannot lie with 30 deg of the plane of the sky. The jet points directly at the base of the kpc-scale jet seen by Perley, Dreher, and Cowan (1984); the curvature occurs primarily or entirely on a scale of 10-100 kpc. The jet brightness falls off more slowly inside 1 kpc than in other powerful sources. A search for compact structure in the three bright hot spots revealed no structure lower than 3 mas in size with flux greater than 6 mJy.

Linfield, R.↗
NASA NTRS1985

Improved mapping of radio sources from VLBI data by least-square fit

A method is described for producing improved mapping of radio sources from Very Long Base Interferometry (VLBI) data. The method described is more direct than existing Fourier methods, is often more accurate, and runs at least as fast. The visibility data is modeled here, as in existing methods, as a function of the unknown brightness distribution and the unknown antenna gains and phases. These unknowns are chosen so that the resulting function values are as near as possible to the observed values. If researchers use the radio mapping source deviation to measure the closeness of this fit to the observed values, they are led to the problem of minimizing a certain function of all the unknown parameters. This minimization problem cannot be solved directly, but it can be attacked by iterative methods which we show converge automatically to the minimum with no user intervention. The resulting brightness distribution will furnish the best fit to the data among all brightness distributions of given resolution.

Rodemich, E. R.↗
NASA NTRS1985

Comparison of GSFC and JPL VLBI modeling software: Benchmark

The Very Long Base Interferometry. The VLBI modeling software packages CALC 6.0 (GSFC) and MASTERFIT (JPL) are compared in some detail. Theoretical model delays are calculated for a set of 120 fictitious observations which involve a variety of sources, baselines, and antennas. Discrepancies between the total delays given by the two programs are of the order of 2 cm (RMS). The modeling of antenna offsets appears to account for approximately half of this difference. Relativistic bending contributions to the delay differ by 3 cm (RMS), and the there appears to be some mutual cancellation of errors involving antenna offsets, bending, and the effects of the two different Solar System ephemerides employed by CALC and MASTERFIT. This cancellation has not been completely characterized here.

Sovers, O. J.↗
NASA NTRS1986

VLBI limits on the proper motion of the 'core' of the superluminal quasar 3C345

VLBI (very-long-baseline interferometry) observations between 1971 and 1983 have been used to determine the positions of the 'core' of the quasar 3C345 relative to the more distant compact quasar NRAO512 with a fractional uncertainty as small as two parts in a hundred million. The core of 3C345 appears stationary in right ascension to within 20 arc microsec/yr, a subluminal bound corresponding to 0.7c. The apparent velocities of the jets are superluminal, up to 14c in magnitude.

Bartel, N.↗
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 NTRS1986

A modified hybrid mapping technique for VLBI data

A modified interferometric mapping technique is presented for observations where absolute phase information is not available. This is especially applicable to VLBI observations. The technique corrects weaknesses of conventional hybrid mapping methods in measuring source symmetry, particularly for arrays with an outrigger antenna. An example of such an array is a set of earth-based antennas plus an orbiting antenna.

Linfield, R. P.↗
NASA NTRS1986

A 43GHz VLBI mapping of SiO maser emission associated with Orion-KL IRC-2

A milliarcsecond resolution spot map of the SiO maser emission associated with IRC-2 in Orion-KL is presented. The two dominant groups of spectral features, near V(LRS) = -6 and 16 km/s, were observed in the 43 GHz, v = 1 to 0 transition of SiO, using a Mark III VLBI system. The 74 km baseline ran from Haystack Observatory in Westford, Massachusetts to Five College Radio Astronomy Observatory (FCRAO) in New Salem, Massachusetts. Five distinct maser features were observed: -8.5 to -6.5 km/s; -5 to -1.5 km/s; 12 to 13.5 km/s; 16.5 to 19 km/s; and 20 to 21 km/s (stellar velocity = 5 km/s). The relative positions were established, from an analysis of fringe phases, to an accuracy of about 5 milliarcseconds. All the features lay within an area of radius 0.08 arcseconds or 6x10(14) cm, at a distance of 500 pc. Previous interferometric studies were only able to measure the gross separation between the red and the blue shifted groups. Our measurement of the separation between these two gropus is consistent with those of the previous studies, indicating the persistence of these two centers of activity. The absolute positions of the masers with respect to IRC-2 are only known to an accuracy of about 1 arcsecond. It is assumed that IRC-2 is centered between the red shifted and the blue shifted maser features. The relative placement of these two groups of maser features agrees with observations of thermal emission from SO, which traces the outflow on a much larger scale. The SiO masers trace the neutral outflow from IRC-2 on the smallest scale yet observed.

Greenhill, L. J.↗
NASA NTRS1986

VLBI observations of the compact components in M82

VLBI observations at 2.3 and 8.4 GHz of 15 compact components in M82, obtained on May 19, 1983, are examined. Angular sizes and linear sizes of the compact sources in M82 are estimated; it is detected that most of the compact sources studied have linear sizes in excess of 0.3 pc. The brightness distribution of the strongest of the 15 compact sources, 41.9 + 58, is investigated; two dominant, elongated components oriented at an angle of about 20 deg and having maximum dimensions of 10-30 mas are observed.

Bartel, N.↗
NASA NTRS1987

Multiepoch VLBI observations of 4C 39.25 - Superluminal motion amid stationary structure

Eight VLBI maps of the quasar 4C 39.25 are presented, covering the time interval November 1979 to July 1985. During this period the compact components at the eastern and western ends of the source remained roughly stationary with respect to each other. A third component emerged from the western component in 1982 and proceeded to move (relative to the other two components) eastward at a rate of 0.16 + or - 0.02 marcsec per year. This corresponds to an average apparent speed between 3.2c and 8.4c for Hubble's constant between 100 and 50 km/s Mpc and q0 between 1 and 0. This superluminal motion contrasts with the stationary structure observed in the 1970s and also still observed between the eastern and western ends of the source. Possible explanations include superluminal feeding of a stationary compact counterpart to a classical radio lobe, an obstacle in a relativistic jet, a relativistic jet which alternatively expands and contracts owing to gradients in the confining pressure, and gravitational lensing of the compact radio emission.

Shaffer, David B.↗
NASA NTRS1986

Creation of a global geodetic network using Mark III VLBI

The positions of 15 permanent VLBI stations have been determined using Mark III with one-sigma uncertainties of less than 5 cm except for three stations in the Pacific. 46070 delay/delay rate observations acquired by the Crustal Dynamics Project and Polaris/IRIS from 1980-84 were included in a least squares solution to estimate the station positions, 44 radio source positions, and earth orientation parameters.

Ma, Chopo↗
NASA NTRS1987

VLBI observations of the 3C 111 jet

VLBI observations have been made of the central component of the broad-line radio galaxy 3C 111 at 1.7 GHz and 5.0 GHz. High-dynamic-range maps at both frequencies show a one-sided jet in PA 62 + or - 1 deg but no counterjet. The lower limit to the jet/counterjet ratio is 30:1 at 5 GHz and 15:1 at 1.7 GHz. The jet is initially straight, but beyond 15 marcsec from the core it exhibits small bends or wiggles. Their onset corresponds to a sharp reduction in the rate of intensity falloff along the jet, suggesting that an instability in the jet is causing substantial particle acceleration.

Linfield, Roger↗
NASA NTRS1988

The Mark III VLBI System

Geodetic measurements have errors in centimeter range. Collection of three reports describes both equipment and results of some measurements taken with Mark III very-long-baseline interferometry (VLBI) system. Has demonstrated high accuracy over short baselines, where phase-delay measurements used. Advanced hardware, called Mark III A, developed to improve system performance and efficiency. Original Mark III hardware and III A subsystem upgrades developed as part of NASA Crustal Dynamics Project at Haystack Observatory.

Rogers, A. E. E.↗
NASA NTRS1987

VLA and low-frequency VLBI observations of the radio source 0503 + 467 - Austere constraints on interstellar scattering in two media

The radio source 0503 + 467 lies near the Galactic plane (l = 161.0 deg, b = 3.7 deg) and at the edge of the supernova remnant (SNR) HB 9. The VLA observations show that it has a spectrum typical of a compact extragalactic radio source. The resultant small angular size of the source makes it an excellent probe of turbulence in two media: the diffuse, or 'type A', component of interstellar turbulence and a hypothesized region of hydromagnetic turbulence upstream of the supernova remnant. An eight-station VLBI experiment at 326 MHz indicates that the source is less than about 20 milliarcseconds (mas) in angular diameter. A value of 16 mas is most appropriate as an upper limit to the interstellar scattering contribution to the measured angular size. The implications of this upper limit are twofold. First, the galactocentric radial scale to the type-A turbulence is probably less than or equal to about 6 kpc. Second, no evidence is seen for shock-associated turbulence upstream of HB 9. The measurements make it possible to constrain a parameter which is a function of the rms density fluctuation in the upstream region, the outer scale to the density turbulence, and the thickness of SNR foreshock region.

Spangler, Steven R.↗
NASA NTRS1987

VLBI observations of 23 hot spots in the starburst galaxy M82

The simultaneous 2.3 and 8.4 GHz VLBI observations of 23 hot spots in the nuclear region of M82 presented indicate the presence of six hot spots at 2.3 GHz, but only one at 8.4 GHz. Attention is given to a mapping of the brightest hot spot, 41.9+58, at 2.3 GHz, which exhibits a complex brightness distribution whose angular width in the NE-SW direction is 45 mas at the 10-percent contour. These data are consistent with the hot spots in M82 being powerful SNRs, with ages between about 10 and 300 yr.

Bartel, Norbert↗
NASA NTRS1988

Dual-frequency and dual-polarization VLBI observations of the stellar system Algol

Results of multiepoch VLBI observations of the stellar system Algol are presented, including dual-polarization and dual-frequency measurements at 2.3 and 8.4 GHz. The brightness temperature of the radio source is generally between 3 x 10 to the 8th K and 5 x 10 to the 9th K and is consistent with gyrosynchrotron emission from energetic electrons with mean energy less than 1 MeV in an active coronal region. Two exceptional events were observed: a high-brightness, broad-band outburst occurring simultaneously at 2.3 and 8.4 GHz, and a short-duration, highly polarized and high-brightness temperature outburst at 1.66 GHz. A simple physical model which includes all three types of emission is suggested.

Lestrade, Jean-Francois↗
NASA NTRS1988

The determination of the trajectory of the motion of a balloon probe in the Venusian atmosphere via VLBI

The trajectories of the motion of balloons floating freely in the Venusian atmosphere (the Vega project) are measured via VLBI. The balloon in the southern hemisphere moved with an average velocity of 66 m/s. The latitudinal velocity component was equal to 3.4 + or - 0.5 m/s and was directed northward. The velocity of the balloon in the northern hemisphere was equal to 69 m/s and was directed nearly parallel to the equator.

Sagdeev, R. Z.↗