M. W. Sinclair

The Subparsec-Scale Structure and Evolution of Centaurus A: The Nearest Active Radio Galaxy

?????The subparsec-scale structure of Cen A is complex, consisting of a bright jet and a fainter counterjet. The bright jet contains components that have subluminal speeds of approximately 0.1c and undergo irregular episodes of rapid internal evolution. The rapid evolution sometimes observed could be interpreted as evidence for an underlying jet flow much faster (>0.45c) than observed from the proper motion of components (~0.1c). Considering the large-scale morphology of the source, the motions and temporal variations in the jet, and the detection of a counterjet, we conclude that the axis of the Cen A jet lies between ~50? and ~80? to our line of sight. We find that the estimated times of component ejection from the compact core are reasonably coincident with enhancements in hard X-ray intensity and 22 GHz flux density. In the context of the radio galaxy population, Cen A is a low-luminosity FR I?type source and in general has the properties observed in other FR I radio galaxies. Overall, the observations of Cen A presented here, and from other investigations, are consistent with the idea that sources with an FR I appearance are not aligned with our line of sight and have relativistic flow on the subparsec scale. The apparently subluminal subparsec-scale jet components are interpreted as being slow patterns on the relativistic flow.

Discovery of a Subparsec Radio Counterjet in the Nucleus of Centaurus A

A subparsec scale radio counterjet has been detected in the nucleus of the closest radio galaxy, Centaurus A (NGC 5128), with VLBI imaging at 2.3 and 8.4 GHz. This is one of the first detections of a VLBI counterjet and provides new constraints on the kinematics of the radio jets emerging from the nucleus of Cen A. A bright, compact core is seen at 8.4 GHz, along with a jet extending along P.A. 51°. The core is completely absorbed at 2.3 GHz. Our images show a much wider gap between the base of the main jet and the counterjet at 2.3 GHz than at 8.4 GHz and also that the core has an extraordinarily inverted spectrum. These observations provide evidence that the innermost 0.4-0.8 pc of the source is seen through a disk or torus of ionized gas which is opaque at low frequencies due to free-free absorption.

PKS 1830–211: A Possible Compound Gravitational Lens

Measurements of the properties of gravitational lenses have the power to tell us what sort of universe we live in. The brightest known radio Einstein ring/gravitational lens PKS 1830-211, while obscured by our Galaxy at optical wavelengths, has recently been shown to contain absorption at the millimeter waveband at a redshift of 0.89. We report the detection of a new absorption feature, most likely due to neutral hydrogen in a second redshift system at z = 0.19. Follow-up VLBI observations have spatially resolved the absorption and reveal it to cover the NE compact component and part of the lower surface brightness ring. This new information, together with existing evidence of the unusual VLBI radio structure and difficulties in modeling the lensing system, points to the existence of a second lensing galaxy along our line of sight and implies that PKS 1830-211 may be a compound gravitational lens.

Extremely Rapid Variations of Water Maser Emission from the Circinus Galaxy

The water maser lines in the Seyfert nucleus of the Circinus galaxy vary on timescales as short as a few minutes. The amplitude of one line more than doubled in ≈ 10 minutes, reaching ≈ 37 Jy, which corresponded to an increase of ≈ 6 L☉, assuming isotropic emission, in a single maser feature on a size scale of about 1 AU, based on light-travel time. Other lines vary by up to about 30% on similar timescales. The variability is at least 2 orders of magnitude more rapid than any observed for other Galactic or extragalactic water masers. The intensity changes cannot be attributed easily to a mechanism of intrinsic fluctuations. The variability may be the result of strong interstellar diffractive scintillation along the line of sight within our Galaxy. This would be the first example of diffractive scintillation for any source at 22 GHz and for any source other than a pulsar. However, only the very shortest timescales for interstellar scintillation, obtained from pulsar observations and scaled to 22 GHz, correspond to the observed maser variability. Alternatively, the intensity changes may be a reaction to fluctuations in compact background or radiative pump sources and thereby may be related to variability of the central engine. The maser spectral features symmetrically bracket the systemic velocity of the galaxy, with components red- and blueshifted by about ±(100-200) km s-1. The spectrum of the Circinus maser is similar in some respects to that of the maser in NGC 4258, which probably traces a molecular disk rotating around a supermassive object. VLBI observations could reveal whether the maser source in the heart of the Circinus galaxy is part of a similar dynamical system.

A 5 GHz Southern Hemisphere VLBI Survey of Compact Radio Sources. II.

We report the results of a 5 GHz Southern Hemisphere snapshot VLBI observation of a sample of blazars. The observations were performed with the Southern Hemisphere VLBI Network plus the Shanghai station in 1993 May. Twenty-three flat-spectrum, radio-loud sources were imaged. These are the first VLBI images for 15 of the sources. Eight of the sources are EGRET (>100 MeV) γ-ray sources. The milliarcsecond morphology shows a core-jet structure for 12 sources and a single compact core for the remaining 11. No compact doubles were seen. Compared with other radio images at different epochs and/or different frequencies, three core-jet blazars show evidence of bent jets, and there is some evidence for superluminal motion in the cases of two blazars. Detailed descriptions for individual blazars are given.

A 78-114 GHz monolithic subharmonically pumped GaAs-based HEMT diode mixer

A W-band subharmonically pumped (SHP) diode mixer is designed for fixed LO frequency operation. It is fabricated on a 4-mil substrate using 0.15 /spl mu/m GaAs PHEMT MMIC process. The on-wafer measurement results show that the conversion loss is about 10 to 14 dB across the W band, as a 10 dBm 48 GHz LO signal is pumped. To our knowledge, this is the state-of-the-art result on low-conversion-loss wideband MMIC SHP diode mixer. The packaged module measurement shows a similar result. Both the simulation and measurement results are shown to be in good agreement.

A Search for H2O Maser Emission in Southern Active Galactic Nuclei and Star-forming Galaxies: Discovery of a Maser in the Edge-on Galaxy IRAS F01063–8034

We report the cumulative results of five surveys for H 2O maser emission at 1.35 cm wavelength in 131 active galactic nuclei (AGNs) and star-forming galaxies, conducted at the Parkes Observatory between 1993 and 1998. We detected one new maser, in the edge-on galaxy IRAS F01063-8034, which exhibits a single ∼0.1 Jy spectral feature at 4282 ± 6 km s -1 (heliocentric) with an unusually large 54 ± 16 km s -1 half-power full width. The centroid velocity of the emission increased to 4319.6 ± 0.6 km s -1 (38 ± 2 km s -1 width) over the 13 days between discovery and confirmation of the detection. A similarly broad-line width and large change in velocity has been noted for the maser in NGC 1052, wherein jet activity excites the emission. Neither optical spectroscopy, radio-infrared correlations, nor infrared colors provide compelling evidence of unusual activity in the nucleus of IRAS F01063-8034. Since the galaxy appears to be outwardly normal at optical and infrared wavelengths, detection of an H 2O maser therein is unique. The maser emission is evidence that the galaxy harbors an AGN that is probably obscured by the edge-on galactic disk. The detection highlights the possibility that undetected AGNs could be hidden in other relatively nearby galaxies. No other maser emission features have been identified at velocities between 3084 and 6181 km s -1.

Application of commercial antennas to very long baseline interferometry radio astronomy

Groundstation antennas located in Western Australia have been used as elements in the Southern Hemisphere very long baseline interferometry (VLBI) experiment. These commercial antennas of 27.5 and 15 m diameter provide a very economical improvement to the imaging capability of this array and the practice of using such antennas can be extended to other sites in the Southern Hemisphere in preparation for the forthcoming space‐VLBI experiments.

The Southern Hemisphere Contribution to the VSOP Mission

Abstract The Space VLBI mission, VSOP, involves the participation of 15 countries with up to 40 radio telescopes spread across the Earth observing in close coordination with the 8 m radio telescope on board the Japanese spacecraft HALCA (Hirabayashi, 1998, and this Proceedings). This paper describes the contributions to the missions success from Southern Hemisphere radio telescopes, facilities and Institutes.

VLBI imaging of GRO J1655-40 with the sheve array

Abstract On 27 July 1994 the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory detected an outburst of high energy X-rays from a previously unknown source in the constellation Scorpius. This source was designated GRO J1655-40. Approximately 12 days after the start of the X-ray outburst, a strong outburst of radio emission occurred. Here we present very long baseline interferometry (VLBI) observations of GRO J1655-40 made with an array of telescopes in Australia, South Africa, and the western United States. These observations show that the radio source which appeared two weeks after the initial X-ray outburst consisted of two prominent components which separated with an apparent speed of 1.5 ± 0.4c. When the various possibilities for the geometry of the radio source are taken into account the apparent speed implies an intrinsic speed between 0.5c and 0.9c. Our results and those of other investigators imply a strong link between the accretion of material onto a highly compact object and the ejection of relativistic components of radio emission.