VLBI Array Research Articles

VLBI observations of the water vapor maser in the nucleus of NGC 3079

view Abstract Citations (44) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLBI Observations of the Water Vapor Maser in the Nucleus of NGC 3079 Haschick, A. D. ; Baan, W. A. ; Schneps, M. H. ; Reid, M. J. ; Moran, J. M. ; Guesten, R. Abstract The water vapor (H2O) maser source in the nucleus of NGC 3079 has been observed with a four-station intercontinental VLBI array, which had a resolution of 0.3 mas. The angular separation among the individual maser features is ~ 0.3 mas (7 x 10^16^ cm), but individual features are unresolved and smaller than 0.06 mas (1.5 x 10^16^ cm). We also made VLA observations of NGC 3079, which show that the maser source is coincident in angle with the compact continuum radio source in the nucleus of the galaxy to better than 5 mas. The water vapor maser source in NGC 3079 is comparable in size, but apparently 500 times more luminous than the brightest H_2_O maser source in the Galaxy, W49N, based on the assumption of isotropic emission. The small size and apparent high luminosity of this extragalactic source suggests special circumstances for the maser. A pumping model with many OB stars confined to such a very small volume is implausible. We propose a model wherein the maser is an unsaturated amplifier of the nuclear continuum source. The maser has a low gain of ~ 80, and the maser emission is very highly beamed. The true luminosity of a single masing cloud with its radiation beamed toward us can be ~10^-7^ L_sun_. The pumping requirements for a single cloud, ~ 10^48^ pump photons s^-1^, are about the same as for a strong maser in our Galaxy. Based on probability arguments for detecting extragalactic masers, the total luminosity of all masers in NGC 3079 can be on the order of 10-50 L_sun_. Publication: The Astrophysical Journal Pub Date: June 1990 DOI: 10.1086/168825 Bibcode: 1990ApJ...356..149H Keywords: Galactic Nuclei; Interstellar Masers; Very Long Base Interferometry; Water Masers; Luminosity; Power Spectra; Star Formation; Water Vapor; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 3079; GALAXIES: INTERSTELLAR MATTER; GALAXIES: NUCLEI; MASERS full text sources ADS | data products SIMBAD (11) NED (1)

Phase-referenced VLBI observations of weak radio sources - Milliarcsecond position of Algol

The Fourier inversion of all the delay/rate referenced visibilities of Algol was used to form a 'dirty' map which has dramatically improved VLBI array sensitivity. The Algol system, in a very low state of activity with a total flux density of only 3 mJy, was observed alternatively with the reference source 0309 + 411, about 1 deg away, throughout the 7 hr VLBI experiment. By combining all the individual VLBI observations, the resulting coherent integration time for Algol is 4.2 hr and the corresponding SNR is 12. Internal consistency checks and an analysis of possible systematic astrometric errors indicate that the accuracy of the relative VLBI position of Algol is + or - 0.0005 arcsec in both coordinates and that unknown systematic effects are presently limiting this astrometric measurement.

Detection of superluminal motion in the core-dominated quasar 0106 + 013

view Abstract Citations (13) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detection of Superluminal Motion in the Core-dominated Quasar 0106+013 Wehrle, Ann E. ; Cohen, Marshall H. ; Unwin, Stephen C. Abstract We have observed the core-dominated quasar 0106+013 with a global VLBI Array at γ = 6 cm in 1986 and 1988. The radio structure changed dramatically between the two epochs from barely resolved to well-separated double with flux ratio 3:2. Identifying the changes with east-west movement between two components, we find the proper motion μ = 0.20 +/- 0.5 mas yr^-1^ which corresponds to v/c = (8.2 +/- 2.0)/h (for z = 2.107; H_0_ = 100 km s^-1^ Mpc^-1^ and q_0_ = 0.5). The speed is typical of superluminal quasars. We note that 0106+013 is a key object in VLBI-based geodetic measurements; knowledge of its structure should improve the accuracy of measurements of crustal dynamics. Publication: The Astrophysical Journal Pub Date: March 1990 DOI: 10.1086/185665 Bibcode: 1990ApJ...351L...1W Keywords: Quasars; Radio Sources (Astronomy); Celestial Mechanics; Cross Correlation; Red Shift; Astrophysics; QUASARS; RADIO SOURCES: GALAXIES; RELATIVITY full text sources ADS | data products SIMBAD (3) NED (1)

Superluminal motion of the radio core of BL Lacertae - 1980.4-1988.3

A VLBI array mapping of the compact radio source associated with BL Lacertae at 5.0 and 10.6 GHz has led to the identification of four superluminally moving components, each of which coincides with a flux and polarization outburst, as well as a nearly stationary knot which persisted during a prolonged period of quiescence. These observations are discussed in terms of relativistic beam models with propagating shocks; it is established that the observations are consistent with models with a Lorentz factor of about 4 and a viewing angle between the observer's line-of-sight and the jet axis of 19-31 deg. 54 refs.

International VLBI Satellite (IVS)

AbstractIVS is under study in ESA as a second generation space VLBI observatory. The mission concept calls for a 25 m diameter radio telescope in space funded by the principal space agencies. Orbiting the Earth and observing in concert with the established ground-based VLBI arrays in Europe, USA, USSR and Australia, IVS will provide high quality images of galactic and extragalactic radio sources at wavelengths spanning the radio band from decimetres to millimetres with resolution as high as 10 micro arcseconds and sensitivity equal to those of ground-based images. New features of IVS compared to the first generation missions are: a more than order of magnitude increase in sensitivity; an order of magnitude increase in maximum angular resolution; extension of the wavelength range to the millimetre band; and the capability to operate as a stand-alone radio telescope enabling it to explore new frontiers in spectral line and microwave background research, in particular the distribution of galactic molecular oxygen and Compton scattering of the microwave background by foreground cluster gas.

First direct geodetic link between Europe, Africa and South America with a multi station VLBI array

The first geodetic experiment tying Europe, Africa and South-America was realized in July 1985 by Very Long Baseline Interferometry with a network of 5 radiotelescopes. TheVLBI technique and data analysis are presented, with special emphasis on the ionosphere modeling because of its importance in this particular experiment. Comparisons of the results with other geodetic information confirm the achievement of decimetric accuracy.

The Southern Hemisphere VLBI experiment

Six radio telescopes were operated as the first Southern Hemisphere VLBI array in April and May 1982. Observations were made at 2.3 and 8.4 GHz. This array provided VLBI modeling and hybrid imaging of celestial radio sources in the Southern Hemisphere, high-accuracy VLBI geodesy between Southern Hemisphere sites, and subarcsecond radio astrometry of celestial sources south of declination -45 deg. The goals and implementation of the array are discussed, the methods of modeling and hybrid image production are explained, and the VLBI structure of the sources that were observed is summarized.

A fast-moving knot of radio emission in the active galaxy NGC 1275

view Abstract Citations (26) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Fast-moving Knot of Radio Emission in the Active Galaxy NGC 1275 Marr, Jonathan M. ; Backer, Donald C. ; Wright, Mel C. H. ; Readhead, Anthony C. S. ; Moore, Richard Abstract We have mapped the nearby (z = 0.018) active galaxy NGC 1275 (3C 84) at six different epochs from 1981 to 1986 at 1.3 cm (22.3 GHz) with a global VLBI array of seven telescopes. In this paper we describe a long-lived knot of emission separating from the main radio component with a projected velocity of 0.49 +/- 0.10h^-1^c. This knot moves through diffuse emission that also moves away from the main component with a slower projected velocity of 0.33 +/- 0.13h^-1^c. We show that the knot and diffuse emission are associated with flux-density increases that occurred around 1968 and 1959, respectively. We discuss the observations in terms of a core-jet model and propose that the jet collimation has increased over the lifetime of the present activity. Publication: The Astrophysical Journal Pub Date: February 1989 DOI: 10.1086/167138 Bibcode: 1989ApJ...337..671M Keywords: Active Galaxies; Peculiar Galaxies; Radio Sources (Astronomy); Astronomical Models; Flux Density; Very Long Base Interferometry; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 1275; GALAXIES: NUCLEI; GALAXIES: SEYFERT; GALAXIES: STRUCTURE; INTERFEROMETRY; RADIO SOURCES: GALAXIES full text sources ADS | data products SIMBAD (1) NED (1)

VLBI and MERLIN observations of 3C 120 at 1.7 GHz - Superluminal motions beyond 0.05 arcsec

The radio galaxy 3C 120 was observed at 1.7 GHz in 1982 and 1984 with VLBI arrays, and the results are reported. Significant changes in the radio structure are seen on scales out to 100 mas between two observations separated by 1.5 yr. The changes indicate the presence of superluminal motions at 50 mas and probably at 90 mas from the core. These are the largest angular scales on which superluminal motions have been observed. The continuity of the 3C 120 jet on scales of a fraction of an arcsec is established. The observations that the jet does not slow down over an order of magnitude in core distance, combined with the continuity of the physical parameters deduced from observations over many orders of magnitude in core distance, suggests that the jet remains relativistic on even larger scales. This supports the concept that the powerful, one-sided jets are relativistic on all scales and that the one-sidedness is the result of relativistic beaming. 31 references.

VLBI observations of the gravitational lens system 0957+561 - Structure and relative magnification of the A and B images

view Abstract Citations (54) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLBI Observations of the Gravitational Lens System 0957+561: Structure and Relative Magnification of the A and B Images Gorenstein, M. V. ; Cohen, N. L. ; Shapiro, I. I. ; Rogers, A. E. E. ; Bonometti, R. J. ; Falco, E. E. ; Bartel, N. ; Marcaide, J. M. Abstract We observed the putative gravitationally lensed images of the quasar, 0957+561, at 13 cm wavelength with a six-antenna VLBI array having nearly milliarcsecond resolution. Using four elliptically shaped Gaussian components to model the observed surface brightness distribution of each of the A and B images, we found that the resultant models offer strong evidence in support of the gravitational lens hypothesis. A simple, 2 x 2 magnification matrix relates quantitatively the brightness distributions of the two "long" jet components in one image to those in the other. The flux density, size, shape, and orientation of these two jet components, one each of length ~10 and 50 milliarcsecond, are likely to change only on a time scale long compared to the difference in light propagation times from source to observer ("time delay") for the two images. Thus the magnification matrix provides a value for the magnitude of the relative image magnification, B/A, of 0.64 +/- 0.03; in addition, the elements of the matrix can be used to constrain models of the mass distribution of the lens. The ratio, B/A, of the flux density of the ~ 1 milliarcsecond diameter core components in one image to that in the other is 0.82 +/- 0.02, apparently inconsistent with the relative magnification determined for the long jet components. This apparent inconsistency is evidence for changes in the brightness distribution of the quasar core, on a time scale comparable to, or shorter than, the time delay. Thus continued monitoring with VLBI of the flux density in each component of each image might provide a reliable determination of this time delay. Our analysis of the differences of the interferometer phases associated with these images yielded the difference in position between their brightest components, with a standard error of about 20 microarcsecs. Publication: The Astrophysical Journal Pub Date: November 1988 DOI: 10.1086/166816 Bibcode: 1988ApJ...334...42G Keywords: Gravitational Lenses; Quasars; Very Long Base Interferometry; Astronomical Models; Brightness Distribution; Calibrating; Astronomy; GRAVITATIONAL LENSES; INTERFEROMETRY; QUASARS full text sources ADS | data products SIMBAD (4) NED (3)

Structural Variability in the Core of 3C147

We have observed the core of the steep spectrum compact quasar 3C147 at a wavelength of 6 cm with an intercontinental VLBI array. The three observing epochs cover a period from 1981.25 to 1984.8. The core of 3C147 exhibits a complex, nonlinear, and asymmetric structure. It shows structural variability as well as variations of individual components. The observed changes in separation corespond to an apparent velocity of 2.6 ±1.0 c (H0= 50 km s−1Mpc−1, q0= 0.5). In addition a map from the 3-station observation in 1978 is presented.

Kinematic Properties of the Ejected Matter in NGC 1275

We have mapped the nearby (z=0.018), active galaxy NGC 1275 (3C84) at 6 different epochs from 1981 to 1986 at 1.3 cm (22.3 GHz) with a global VLBI array of seven telescopes. We find a long-lived knot of emission separating from the brightest radio component with a projected velocity = 0.4 6±0.12 h−1 c. This knot moves through diffuse emission that also moves away from the main component with a slower projected velocity of 0.33±0.12 h−1 c. We show that the knot and diffuse emission result from two separate events that occurred around 1959 and 1968.

The Southern Hemisphere VLBI Experiment (SHEVE)

Six radio telescopes were operated as the first southern hemisphere VLBI array in April and May 1982. Observations were made at 2.3 and 8.4 Ghz. This array produced VLBI images of 28 southern hemisphere radio sources, high accuracy VLBI geodesy between southern hemisphere sites, and subarcsecond radio astrometry of celestial sources south of declination −45 degrees. This paper discusses only the astrophysical aspects of the experiment.

FFT Processor as a Digital Lens in Grid Array VLBI

Correlator type processors have been currently used in VLBI, in which the number of correlators increases as N(N-1)/2 where N is the number of antennas. In a large elements VLBI array in future, this will introduce difficulty in constructing the processor. If we took the grid array configuration of antenna, FFT processor could be used in VLBI processing instead. FFT processor is used as a digital lens for a directional finding facility of the arrival waves. In FFT, the number of multiplications increases only as N log N. Although it gives maximum redundancy of Fourier components, the next step of the research on crustal motion will require measuring the velocities as a vector fields. Higher S/N ratio is also obtained in the VLBI maps.The present processor, which was originally constructed for a pilot system of the radio patrol camera at Waseda University, is formed by a FFT Processor and eight Complex Amplitude Equalizers(1,2). When it is used as an VLBI processor, phase and amplitude fluctuations due to local oscillators and/or propagation effects are absorbed by the quick control of the Complex Amplitude Equalizers.Here, we show examples of the computer controlled phase adjustment using the Equalizers(3). Our telescope is an 8 elements Very Short Baseline Interferometer (only 2m) at 10 GHz. Fig.1 is the block diagram of the present system. In VLBI system the recording facilities should exist between A/Ds and the Equalizers. The FFT processor is an error free ideal lens, and from the sampling theorem it makes eight independent beams in our system. Fig.2 is the dirty beams before removing phase errors, and Fig.3 the beams after the error removed. Both are 180 deg beam switched, while non switched in Fig.4.

Initial Radio Observations of SN1987a in the Large Magellanic Cloud

A prompt radio burst has been observed from the supernova 1987a in the Large Magellanic Cloud. Observations were made at 0.843, 1.415, 2.29, and 8.41 GHz. At frequencies around 1 GHz, the peak flux density reached about 150 mJy and occurred within four days of the supernova. This event may be a weak precursor to a major radio outburst of the type previously observed in other extragalactic supernovae. Radio monitoring of the supernova is continuing at each of the above frequencies, and coordination is underway of a southern hemisphere VLBI array to map the radio outburst region as it expands. Differential astrometry carried out on prime-focus plates taken with the Anglo-Australian telescope indicates that the component, star 1, of Sanduleak's star SK-69202 is within 0.05 ± 0.13 arcsec of the supernova.

High Resolution Observations of Low frequency Variables at 932 MHz

Fifteen sources known to be varying at low frequencies have been observed at six epochs during 1983–84 with a global VLBI array. Some of the sources show structural variations similar to the superluminals. Beaming effect may therefore play an important role at low frequencies as well as at higher.

92 centimeter VLBI structure of the elliptical galaxy MCG 5-4-18

The 92 cm continuum emission from the nucleus of the elliptical galaxy MCG 5-4-18 (0116+319) was mapped using a seven antenna VLBI array. The emission shows two components separated by about 0.07 arcsec along P.A about 115 deg. Analysis of Perley's (1982) 20 and 6 cm VLA data shows no evidence for high surface brightness features offset from the nucleus by 1-50 kpc. Possible causes of the lack of extranuclear emission are suggested. A total radio spectrum is presented, which shows that the steep-spectrum behavior between 11 and 6 cm does not extend to shorter wavelengths because of the presence of a very compact, time-variable component. The nuclear activity in this elliptical galaxy may be triggered by an interaction with its close binary companion, MCG 5-4-17. 41 references.

High dynamic range VLBI observations of NGC 6251

The large radio galaxy NGC 6251 has been observed with an 11-telescope VLBI array, including the full VLA, at 18 cm. High-dynamic-range maps of both the VLBI-scale and VLA-scale radio structure from these observations, as well as hybrid maps from earlier VLBI experiments at 6 and 13 cm, are presented. The 18-cm VLBI hybrid map has an unusually high dynamic range of about 300 to 1 (ratio of the peak brightness to five times the rms noise level) and is limited by thermal noise rather than calibration errors. A lower limit of 80 to 1 for the ratio of jet/counterjet brightness at parsec scales can be set. This is the strongest such limit produced by VLBI observations of any source to date. If the absence of a detectable counterjet is caused by relativistic beaming, then the jet must be aligned within 45 deg of the line of sight. The VLA map shows a complex structure along the previously detected kiloparsec-scale counterjet. The physical conditions in the inner few parsecs of the NGC 6251 jet and their relation to the conditions in the extraordinarily long and well collimated kiloparsec-scale jets are discussed, and it is shown that both small-scale and large-scale morphology can be explained in terms of a simple model and relativistic beaming.

Observations of Galactic and Extragalactic Jets

Since nearly all discrete radio sources of astronomical interest are of insufficient angular extent for their detailed structural properties to be accessible to single-dish radio telescopes, radio interferometry must be employed to gain information on the morphologies of these objects. Recently constructed imaging interferometer arrays which employ the technique of Fourier synthesis, particularly MERLIN and the VLA (Very Large Array), and the more recent VLBI arrays, have given unprecedented imaging capabilities, with the result that our knowledge, and hence perceptions, of discrete radio sources have vastly changed over the last few years. An equally important parallel development has been image processing algorithms. These have vastly improved the quality of information produced by these arrays, so that an instrument such as the VLA can now produce images with speed and quality exceeding original design specifications by factors of 100 to 1000.

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.