VLBI Array Research Articles

Coordinated Millimeter VLBI Array Observations of R Cassiopeiae: 86 GH[CLC]z[/CLC] S[CLC]i[/CLC]O Masers and Envelope Dynamics

Coordinated Millimeter VLBI Array measurements at 86 GHz have imaged the silicon monoxide masers of the nearby red giant R Cassiopeiae at two epochs separated by approximately 1/12 the stellar pulsation period. The v = 1, J = 2 → 1 transition comes from a ring at 1.5 to 2 stellar radii. Doppler velocity patterns are ordered over scales up to ~1 AU but are not consistent with simple models such as global rotation. One major masing region disappeared in the interval between the two epochs.

86 GH[CLC]z[/CLC] Very Long Baseline Polarimetry of 3C 273 and 3C 279 with the Coordinated Millimeter VLBI Array

Very long baseline polarimetry at 86 GHz probes magnetic field structures within the cores of active galactic nuclei at higher angular resolutions and a spectral octave higher than previously achievable. Observations of 3C 273 and 3C 279 taken in 2000 April with the Coordinated Millimeter VLBI Array have resulted in the first total intensity (Stokes I) and linear polarization VLBI images reported of any source at 86 GHz. These results reveal the 86 GHz electric vector position angles within the jets of 3C 273 and 3C 279 to be orthogonal to each other and the core of 3C 273 to be unpolarized. If this lack of polarization is due to Faraday depolarization alone, the dispersion in rotation measure is 90,000 rad m-2 for the core of 3C 273.

Global very long-baseline interferometry observations of compact radio sources in M82

Observations of the starburst galaxy, M82, have been made with a 20-station global VLBI array at �18cm. Maps are presented of the brightest young supernova remnants (SNR) in M82 and the wide-field mapping techniques used in making images over a field of view of �1 arcminute with 3 milliarcsecond resolution are discussed. A limit has been placed on the power law deceleration of the young SNR, 43.31+592 with an index greater than 0.73 ± 0.11 from observations with the European VLBI Network. Using the global array we have resolved compact knots of radio emission in the source which, with future global observations, will enable better constraints to be placed on the expansion parameters of this SNR. The latest global observations have also provided high resolution images of the most compact radio source in M82, 41.95+575. We determine an upper limit to the radial expansion rate along the major axis of 2000 km s 1 . However, the new images also show structure resembling that of collimated ejection which brings into question the previous explanation of the source as being due to the confinement of a supernova by a high density circumstellar medium. It is apparent that we are now able to image the brightest supernova remnants in M82 with a linear scale which allows direct comparison with galactic SNR such as Cassiopeia A.

Multiline Study with VLBI of SiO Masers in Evolved Stars

We present preliminary results of our observations of the 7 mm (v=1 and v=2 J=1-0) and 3 mm (v=1 J=2-1) SiO masers in the circumstellar envelopes of evolved stars. The 7mm masers were observed with the Very Large Baseline Array (VLBA) and the European VLBI Network (EVN), and the 3mm ones with the Coordinated Millimeter VLBI Array (CMVA). We find similarities between the maps, despite the non-coincidence in the location of most spots in the different 7 mm lines. The visibilities found for most compact spots are compatible with maser sources as small as 0.13 mas (or 0.023 AU at the distance of R Cas).

Multifrequency VLBI observations of faint gigahertz peaked spectrum sources

We present the data and analysis of VLBI observations at 1.6, 5 and 15 GHz of a sample of faint Gigahertz Peaked Spectrum (GPS) sources selected from the Westerbork Northern Sky Survey (WENSS). The 5 GHz observations involved a global array of 16 stations and yielded data on the total sample of 47 sources. A subsample of 26 GPS sources with peak frequencies > 5 GHz and/or peak flux densities > 125 mJy was observed with the VLBA at 15 GHz. A second subsample of 29 sources, with peak frequencies <5 GHz, was observed at 1.6 GHz using a 14 station global VLBI array. In this way, 44 of the 47 sources (94%) in the sample were observed above and at or below their spectral peak. Spectral decomposition allowed us to identify 3, 11, 7, and 2 objects as compact symmetric objects, compact doubles, core-jet and complex sources respectively. However, many of the sources classified as compact double or core-jet sources show only two components making their classification rather tentative. This may explain why the strong morphological dichotomy of GPS quasars and galaxies found for radio-bright GPS sources, is not as clear in this faint sample.

J-Net Galactic-Plane Survey of VLBI Radio Sources for VLBI Exploration of Radio Astrometry (VERA)

In order to search for new VLBI sources in the galactic plane that can be used as phase reference sources in differential VLBI, we conducted 22 GHz observations of radio sources in the galactic plane using the Japanese VLBI Network (J-Net). We have observed 267 VLBI source candidates selected from existing radio surveys and have detected 93 sources at a signal-to-noise ratio larger than 5. While 42 of the 93 detected sources had already been detected with VLBI at relatively lower frequency (typically 2 to 8 GHz), the remaining 51 are found to be new VLBI sources detected for the first time. These are located within |b| ≤ 5°, and have a large number of galactic maser sources around them. Thus, they are potential candidates for phase reference sources for VLBI Exploration of Radio Astrometry (VERA), which is the first VLBI array dedicated to phase-referencing VLBI astrometry aimed at measuring the parallax and proper motion of maser sources in the whole Galaxy.

86 GH[CLC]z[/CLC] S[CLC]i[/CLC]O Masers toward Mira

We observed the v = 1, J = 2 → 1, 86 GHz SiO maser transition from Mira ( Ceti) near stellar pulsation phase = 0.3 with the Coordinated Millimeter VLBI Array. Maser emission was detected on two of the baselines, and general size and brightness parameters were extracted. For this epoch (1998.26), we find that the maser features extend over roughly 1.5 to 2 stellar radii R* in the north-south direction. The limited image fidelity was insufficient to assess whether a ringlike morphology typical of other evolved stars with SiO masers appears in Mira also. The brightness temperatures (≤1010.5 K) and spot sizes (≥0001) generally resemble those of other maser sources measured by VLBI at 86 GHz, such as VX Sgr and R Cas. We find no coherent velocity structure suggested by the 86 GHz SiO masers toward Mira.

A Stationary Core with a One‐sided Jet in the Center of M81

The nucleus of the nearby spiral galaxy M81 was observed at 8.4 GHz with a global VLBI array at 20 epochs over 4.5 yr, with a linear resolution at the source of about 2000 AU, or 0.01 pc. Phase-referenced mapping with respect to the geometric center of SN 1993J in the same galaxy enabled us to find, with a standard error of about 600 AU, a stationary point in the source southeast of the brightness peak. We identify this point as the location of the core and the putative black hole at the gravitational center of the galaxy. The 2 σ upper bound on the core's average velocity on the sky is ≤40 μas yr-1, or ≤730 km s-1 (relative to the center of SN 1993J, in excess of galactic rotation). A short, one-sided jet extends toward the northeast from the core, in projection approximately in line with the rotation axis of the galaxy, and toward the well-known extended emission 1 kpc farther out. The orientation of the jet varies smoothly, with timescales of about 1 yr and an rms of 6° around its mean position angle of 50°. Occasionally the jet appears to bend to the east. The length of the jet is only about 1 mas (3600 AU) and varies with an rms of about 20% from epoch to epoch. The inferred speeds are below 0.08c. The total flux density of the core jet varies erratically, changing on occasion by a factor of 2 over a few weeks, without any significant changes in the source size and orientation. The inferred velocity of the plasma flow is greater than 0.25c. The results are consistent with a model in which plasma condensations with short lifetimes are ejected relativistically from the core on a timescale of less than a few weeks. The condensations travel along a tube whose pattern and geometry are also variable but only on a timescale of about 1 yr. The central engine of M81 has qualitative similarities to those of powerful active galactic nuclei of radio galaxies and quasars and may also represent in power and size a scaled-up version of the largely hidden nucleus in our own Galaxy.

Space VLBI observations of southern hemisphere gamma-ray and non-gamma-ray AGN: First results for PKS 0637-752

Abstract We present the first results from our VSOP proposal, “γ-ray loud and quiet AGN with VSOP and SHEVE at 5 GHz”. This project is aimed at determining what, if anything, differentiates γ-ray-identified radio sources from similar strong, flat-spectrum radio sources which have not been identified as sources of greater than 100 MeV γ-rays. Five VSOP observations have been successfully completed as part of this project during VSOP AO1. One observation has been correlated thus far, a 4.970 GHz observation of PKS 0637-752 from 1997 November 21. We present the VSOP image for PKS 0637-752 and make comparisons between the space VLBI image and ground-based VLBI and Australia Telescope Compact Array (ATCA) images, as well as present ATCA flux density monitoring at 4.8 GHz. A brightness temperature for PKS 0637-752 of 0.7 × 1012 K has been determined from the VSOP data and a tentative apparent superluminal speed, from comparison to the ground-based VLBI image, of ∼9c.

Deep 8.4 GHz VLBI Images of Seven Faint Nuclei in Lobe‐dominated Quasars

We present deep 8.4 GHz VLBI images of the faint nuclei in seven lobe-dominated quasars (LDQs): 3C 175, 3C 204, 3C 205, 3C 215, 3C 351, 0821)447, and 0839)616 . These images, made from data obtained in 1988 and 1991 with high-sensitivity VLBI arrays, have a typical rms noise level of D0.03 mJy per beam and resolution of D0.5 mas. The parsec-scale jet structures are generally greater in extent and complexity than those seen on many previous, less sensitive maps of other LDQ nuclei; the jet in 3C 204 bends through at least 10i, and the jet in 3C 205 extends out to D5 mas. Observations of 3C 204, 3C 205, and 0839)616 at two epochs reveal clear —ux density variability in 3C 204 and 3C 205, but none of these sources shows any evidence for outward motion of jet components; their mean appar- ent transverse velocity is ((0.47 ^ 0.43)h~1c, from which we determine a 3 p upper limit on their mean expansion speed of D0.8 h~1c km s~1 Mpc~1, The innermost jet knot in 3C 204 (H 0 100 hq 0 0.5). underwent a marginal position angle swing away from the mean jet axis. We also present the —rst 8.4 GHz A-array VLA images for all the objects except 3C 351. All the sources except 0821)447 display one-sided VLBI and VLA jets, which lie on the same side of the VLBI core and are generally well aligned (to within D5i). In addition, we analyze the statistics of projected linear size, relative nuclear strength, and misalignment angle for the 3CR and Jodrell Bank LDQ samples. We conclude that our results are generally consistent with a fairly simple relativistic jet model and with the uni—cation of core- dominated quasars, lobe-dominated quasars, and possibly even powerful radio galaxies; there is a mar- ginal suggestion that the quasars in our samples have a restricted range of angles to the line of sight (e.g., 0i to D45i). Subject headings: galaxies: jetsgalaxies: structurequasars: generalradio continuum: galaxies ¨ techniques: interferometric

VLBI observations of 21 extragalactic radio sources

We present the results of VLBI observations of 21 extragalactic radio sources carried out with an ad-hoc VLBI array including Shanghai, Urumqi: Noto, and Hartebeesthoek radio telescopes. Eleven sources are imaged, three of them for the first time with VLBI. Correlated flux densities of the remaining ten sources are given. From comparison with data taken at earlier epochs, the apparent superluminal motion is confirmed in four sources (0420-014. 0552+398, 1334-127 and 2345-167) and two more superluminal candidates (1504-166 and 2243-123) are found. The physical parameters of the four superluminal sources are derived.

VLBI Observations of Gamma‐Ray–quiet Active Galactic Nuclei: Comparing Radio‐Core Brightness Temperatures

We present VLBI and Australia Telescope Compact Array images and derive source-frame radio-core brightness temperatures for three prominent, flat-spectrum extragalactic radio sources, notable because they have not been detected as gamma-ray sources with the EGRET instrument. Radio-core brightness temperatures are discussed as an indicator of relativistic beaming.

Symmetric Parsec-Scale OH Megamaser Structures in Arp 220

The parsec-scale OH megamaser emission in the luminous IR galaxy Arp 220 has been imaged in detail using a global VLBI array. Four major emission regions are revealed in the 1667 MHz line, each with complex spatial and velocity structure showing intriguing symmetries.

Multi-Frequency VLBI Observations of the GPS Source 1934–638

AbstractPKS 1934–638 is an archetypal GPS source, peaking at 1.4 GHz and exhibits almost no flux density variability. VLBI images at frequencies of .843, 2.3, 4.8, &amp; 8.4 were made with the southern hemisphere VLBI array and they reveal that the source is a 42 mas compact double. There is no detectable change in separation over the last 20 years, yielding an upper limit of ~ 0.03c ± 0.2c on any expansion velocity. The spectral shapes of the two components are remarkably similar, despite indications of finer structure on longer baselines. Magnetic field calculations indicate fields of a few mGauss and the results are consistent with equipartition.

VLBI observation of OH masers in G 45.07+0.13

Both the left-and right-handed circularly polarized radiations of the masers in the ultra-compact hydrogen II region G45.07+0.13, due to the transition from the OH ground state 2II 3 2 , J = 3 2 , F = 1 → 1, have been observed with the US VLBI array. using the multiplepoint fringe rate method, we have constructed the map of the maser structure with a relative position accuracy of 20 mas. With the fringe rate of the calibration source and that of the maser component with a velocity of 56.7 km s −1, taken to be the phase reference point, the absolute position of the phase reference point has been calculated as given by α (1950.0) = 19 h11 m00.46 s, δ(1950.0) = 10°45′ 43.2″. We have found that most maser components are located in the front of the comet-shaped HII region and that their distance to the center of hydrogen II region is approximately 0.4″. This agrees well with the distance from the vertex of the parabolic ionization wavefront to the young celestial body, as estimated with the bow shock model. By use of the tube-shaped model and under the assumption of fully saturated radiation, the number density of the hydrogen molecules in maser regions has been derived to be 4.5×10 8cm −3. Such high density regions are very possibly the thick clouds of neutral molecules swept up by shock waves and are located in the front side of ionization wavefront. In this maser active region a pair of Zeemansplit lines have been found and from the difference of their radial velocities a magnetic field of 3 mG has been inferred. Its direction is away from the Earth. Similar to the observation of G34.3±0.2, the diameter of the maser cluster near the compact hydrogen II region has the order of magnitude of 10 16 cm. This is 50 times larger than the thickness of a neutral gas layer possibly swept up by shock waves. It is still not clear how this can be explained.

Imaging of Extragalactic Radio Sources with the VSOP Space VLBI Mission

In September 1996 the first dedicated VLBI spacecraft, VSOP, will be launched. This Japanese spacecraft operating in conjunction with ground-based VLBI arrays such as the EVN or VLBA will enable observers to routinely undertake VLBI observations with maximum baseline lengths of 2.6 Earth diameters and a resolution of 55 μas at the highest operating frequency of 22 GHz. In this paper we present a brief overview of the imaging capability of the VSOP mission together with an example of an imaging simulation.

The distribution of spectral index in the structure of quasar 3C 286

High dynamic range images on mas scale and the distribution of spectral index in the structure of the quasar 3C 286 were obtained from observations of global VLBI arrays at 5 and 1.66 GHz. There are two components located the center of the quasar, but we cannot decide which one is the real core from the spectral index. It is impossible that the structure of 3C 286 is the result of gravitational lensing. There may be two active black holes at the center, i.e., two central engines, each with its own jet extending in different directions.

The compact triples 0710 + 439 and 2352 + 495 - A new morphology of radio galaxy nuclei

view Abstract Citations (48) References (67) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Compact Triples 0710+439 and 2352+495: A New Morphology of Radio Galaxy Nuclei Conway, J. E. ; Pearson, T. J. ; Readhead, A. C. S. ; Unwin, S. C. ; Xu, W. ; Mutel, R. L. Abstract The compact radio sources 0710+439 and 2352+495 have each been mapped using intercontinental VLBI arrays at frequencies of 5.0 and 10.7 GHz. The source 0710+439 has also been mapped at 1.7 GHz. Both sources consist of three principal components that lie almost on a straight line. Except for the existence of three VLBI components instead of two they both seem to have similar properties to members of the compact double class. Multi- epoch observations show that apparent velocities between the components in both sources are below c. In the source 0710+439 comparison of multi- epoch data allows us to set a limit of 0.054+/-0.14 h^-1^ c on the rate of separation of the two brightest components. In 2352+495 the multi- epoch 5 GHz VLBI data show that the flux density variations detected by total flux density monitoring are due to changes within the middle component. These fluctuations may be due to refractive interstellar scintillation or they may be intrinsic to the source; in either case the middle component must contain the most compact structure within the source. The morphology, spectra and variability of the middle components strongly suggest that the center of activity in both sources lies here rather than in either of the outer components, providing the first clear evidence of symmetric radio structure in a powerful active galaxy nucleus. Publication: The Astrophysical Journal Pub Date: September 1992 DOI: 10.1086/171697 Bibcode: 1992ApJ...396...62C Keywords: Active Galactic Nuclei; Active Galaxies; Galactic Structure; Radio Galaxies; Very Long Base Interferometry; Radio Sources (Astronomy); Radio Spectra; Astrophysics; GALAXIES: ACTIVE; GALAXIES: INDIVIDUAL ALPHANUMERIC: 0710; 439; GALAXIES: INDIVIDUAL ALPHANUMERIC: 2352; 495; GALAXIES: NUCLEI; RADIO CONTINUUM: GALAXIES; TECHNIQUES: INTERFEROMETRIC full text sources ADS | data products SIMBAD (10) NED (1)

The Very Long Baseline Array

AbstractThe Very Long Baseline Array, currently under construction, is a dedicated VLBI array consisting of ten 25m diameter antennas and a 20 station correlator. The key design features that make it a flexible, general purpose instrument are reviewed and some details are provided of the array layout, frequency coverage and sensitivity.

The Australia telescope

The Australia Telescope consists of three main components, the Compact Array, located near Narrabri, NSW, the Mopra antenna, located near Coonabarabran, NSW, and the Parkes 64-m antenna. The recently completed Compact Array consists of six 22-m diameter antennas on a 6-km east-west baseline. The Mopra antenna is a new 22-m diameter antenna of similar design to the Compact Array antennas. Each of the components of the Australia Telescope may be used individually or they may be combined as a Long Baseline Array or part of a larger VLBI array. Although the data recording systems for array operation have yet to be defined, there is a commitment to provide compatibility for global and space VLBI.