P. M. McCulloch

A warped accretion disk and wide angle outflow in the inner parsec of the Circinus Galaxy

We present the first VLBI maps of H2O maser emission (λ1.3 cm) in the nucleus of the Circinus galaxy, constructed from data obtained with the Australia Telescope Long Baseline Array. The maser emission traces a warped, edge-on accretion disk between radii of 0.11 ± 0.02 and ~0.40 pc, as well as a wide-angle outflow that extends up to ~1 pc from the estimated disk center. The disk rotation is close to Keplerian (v ∝ r-0.5), the maximum detected rotation speed is 260 km s-1, and the inferred central mass is (1.7 ± 0.3) × 106 M☉. The outflowing masers are irregularly distributed above and below the disk, with relative outflow velocities up to ~±160 km s-1, projected along the line of sight. The flow probably originates closer than 0.1 pc to the central engine, possibly in an inward extension of the accretion disk, although there is only weak evidence of rotation in the outward-moving material. We observe that the warp of the disk appears to collimate the outflow and to fix the extent of the ionization cone observed on larger angular scales. This study provides the first direct evidence (i.e., through imaging) of dusty, high-density, molecular material in a nuclear outflow less than 1 pc from the central engine of a Seyfert galaxy, as well as the first graphic evidence that warped accretion disks can channel outflows and illumination patterns in active galactic nuclei. We speculate that the same arrangement, which in some ways obviates the need for a geometrically thick, dusty torus, may apply to other type 2 active galactic nuclei.

The Vela Pulsar’s Proper Motion and Parallax Derived from VLBI Observations

The Vela pulsar is the brightest pulsar at radio wavelengths. It was the object that told us (via its glitching) that pulsars were solid rotating bodies not oscillating ones. Along with the Crab pulsar, is it the source of many of the models of pulsar behavior. Therefore it is of vital importance to know how far away it is and its origin. The proper motion and parallax for the Vela pulsar have been derived from 2.3 and 8.4 GHz very long baseline interferometry (VLBI) observations. The data span 6.8 years and consist of 11 epochs. We find a proper motion of μα cos δ = -49.68 ± 0.06, μδ = 29.9 ± 0.1 mas yr-1 and a parallax of 3.5 ± 0.2 mas, which is equivalent to a distance of 287 pc. When we subtract out the Galactic rotation and solar peculiar velocity, we find μ* = 45 ± 1.3 mas yr-1 with a position angle of 301° ± 18 which implies that the proper motion has a small but significant offset from the X-ray nebulas symmetry axis.

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.

A Radio Reference Frame

A catalogue is presented based on the radio positions of 436 extragalactic sources distributed over the entire sky. The positional accuracy of the sources is better than 3 milliarcsec (mas) in both coordinates, with the majority of the sources having errors better than 1 mas. This catalogue is based upon a general solution of all applicable dual frequency 2.3 and 8.4 GHz Mark-III VLBI data available through the end of 1993 consisting of 1,015,292 pairs of group delay and phase delay rate observations. Details and positions are also given for an additional 124 objects that either need further observation or are currently unsuitable for the definition of a reference frame. The final orientation of the catalogue has been obtained by a rotation of the positions into the system of the International Earth Rotation Service and is consistent with the FK5 J2000.0 optical system, within the limits of the link accuracy.

Methanol Masers as Tracers of Circumstellar Disks

We show that in many methanol maser sources the masers are located in lines, with a velocity gradient along them which suggests that the masers are situated in edge-on circumstellar, or protoplanetary, disks. We present VLBI observations of the methanol maser source G309.92 + 0.48, in the 12.2 GHz (2 0-3 -1 E) transition, which confirm previous observations that the masers in this source lie along a line. We show that such sources are not only linear in space but, in many cases, also have a linear velocity gradient. We then model these and other data in both the 6.7 GHz (5 1-6 0 A +) and the 12.2 GHz (2 0-3 -1 E) transition from a number of star formation regions and show that the observed spatial and velocity distribution of methanol masers, and the derived Keplerian masses, are consistent with a circumstellar disk rotating around an OB star. We consider this and other hypotheses and conclude that about half of these methanol masers are probably located in edge-on circumstellar disks around young stars. This is of particular significance for studies of circumstellar disks because of the detailed velocity information available from the masers.

The Time Delay in the Gravitational Lens PKS 1830–211

We have measured a time delay of 26+ 4−5 days and a magnification ratio of 1.52±0.05 in the strong radio gravitational lens PKS 1830-211. Observations were made over the 18 month period from 1997 January to 1998 July with the Australia Telescope Compact Array at 8.6 GHz, and they have shown that the source started a large flux density outburst around 1997 June.

A survey of the Galactic plane for 6.7-GHz methanol masers — I. l = 325°−335°; b= − 0°.53−0°.53

We report the results of the first complete survey of an area of the Galactic Plane for maser emission from the 6.7-GHz transition of methanol. The survey covers a 10.6-square-degree region of the Galactic Plane in the longitude range 325-335 degrees and latitude range -0.53-0.53 degrees. The survey is sensitive to masers with a peak flux density greater than approximately 2.6 Jy. The weakest maser detected has a peak flux density of 2.3 Jy and the strongest a peak flux density of 425 Jy. We detected a total of 50 distinct masers, 26 of which are new detections. We show that many 6.7-GHz methanol masers are not associated with IRAS sources, and that some are associated with sources that have colours differing from those of a typical ultra-compact HII region (UCHII). We estimate that the number of UCHII regions in the Galaxy is significantly more than suggested by IRAS-based estimates, possibly by more than a factor of two.

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.

A Strong Jet-Cloud Interaction in the Seyfert Galaxy IC 5063: VLBI Observations

We present 21 cm H I line and 13 cm continuum observations, obtained with the Australian Long Baseline Array, of the Seyfert 2 galaxy IC 5063. This object appears to be one of the best examples of Seyfert galaxies where shocks produced by the radio plasma jet influence both the radio and the near-infrared emission. The picture resulting from the new observations of IC 5063 confirms and completes the one derived from previous Australia Telescope Compact Array (ATCA) lower resolution observations. A strong interaction between the radio plasma ejected from the nucleus and a molecular cloud of the interstellar medium (ISM) is occurring at the position of the western hot spot, about 0.6 kpc from the active nucleus. Because of this interaction, the gas is swept up, forming around the radio lobe a cocoon-like structure where the gas is moving at high speed. Because of this, part of the molecular gas is dissociated and becomes neutral or even ionized if the UV continuum produced by the shocks is hard and powerful enough. In the 21 cm H I line new data, we detect only part of the strong, blueshifted H I absorption that was previously observed with ATCA at lower resolution. In particular, the main component detected in the VLBI absorption profile corresponds to the most blueshifted component in the ATCA data, with a central velocity of 2786 km s-1 and therefore blueshifted ~614 km s-1 with respect to the systemic velocity. Its peak optical depth is 5.4%. The corresponding column density of the detected absorption, for a spin temperature of 100 K, is NH I ~ 2 ? 1021 atoms cm-2. Most of the remaining blueshifted components detected in the ATCA H I absorption profile are now undetected, presumably because this absorption occurs against continuum emission that is resolved out in these high-resolution observations. The H I absorption properties observed in IC 5063 appear different from those observed in other Seyfert galaxies, where the H I absorption detected is attributed to undisturbed foreground gas associated with the large-scale galaxy disk. In the case of IC 5063, only a small fraction of the absorption can perhaps be due to this. The reason for this could be that the western jet in IC 5063 passes through a particularly rich ISM. Alternatively, because of the relatively strong radio flux produced by this strong interaction and the high spectral dynamic range of our observations, broad absorption lines of low optical depth as detected in IC 5063 may have remained undetected in other Seyfert galaxies that are typically much weaker radio emitters or for which existing data are of poorer quality.

High Time Resolution Observations of the January 2000 Glitch in the Vela Pulsar

Pulsars are rotating neutron stars, sweeping the emission regions from the magnetic poles across our line of sight. Isolated neutron stars lose angular momentum through dipole radiation and (possibly) particle winds; hence, they slow down extremely steadily, making them among the most reliable timing sources available. However, it is well known that younger pulsars can suffer glitches, when they suddenly deviate from their stable rotation period. On 2000 January 16 (MJD 51,559), the rate of pulsation from the Vela pulsar (B0833-45) showed such a fractional period change of 3.1 × 10-6, the largest recorded for this pulsar. The glitch was detected and reported by the Hobart radio telescope. The speedy announcement allowed the X-ray telescope, Chandra, and others to make Target-of-Opportunity observations. The data placed an upper limit of 40 s for the transition time from the original to the new period. Four relaxation timescales are found, which are believed to be due to the variable coupling between the crust and the interior fluid. One is very short, about 60 s; the others have been previously reported and are 0.56, 3.33, and 19.1 days in length.