Richard Dodson

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.

Science with the Australian Square Kilometre Array Pathfinder

The future of cm and m-wave astronomy lies with the Square Kilometre Array (SKA), a telescope under development by a consortium of 17 countries that will be 50 times more sensitive than any existing radio facility. Most of the key science for the SKA will be addressed through large-area imaging of the Universe at frequencies from a few hundred MHz to a few GHz. The Australian SKA Pathfinder (ASKAP) is a technology demonstrator aimed in the mid-frequency range, and achieves instantaneous wide-area imaging through the development and deployment of phased-array feed systems on parabolic reflectors. The large field-of-view makes ASKAP an unprecedented synoptic telescope that will make substantial advances in SKA key science. ASKAP will be located at the Murchison Radio Observatory in inland Western Australia, one of the most radio-quiet locations on the Earth and one of two sites selected by the international community as a potential location for the SKA. In this paper, we outline the ASKAP project and summarise its headline science goals as defined by the community at large.

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.

Methanol and water masers in IRAS 20126+4104: the distance, the disk, and the jet

Context. Knowledge of the distance to high-mass star forming regions is crucial to obtain accurate luminosity and mass estimates of young OB-type (proto)stars and thus better constrain their nature and age. IRAS20126+4104 is a special case, being the best candidate of a high-mass (proto)star surrounded by an accretion disk. Such a fact may be used to set constraints on theories of highmass star formation, but requires confirmation that the mass and luminosity of IRAS20126+4104 are indeed typical of a B0.5 star, which in turn requires an accurate estimate of the distance. Aims. The goal of our study is twofold: to determine the distance to IRAS20126+4104, using the parallax of H2O masers associated with the source, and unveil the 3D velocity field of the disk, through proper motion measurements of the 6.7 GHz CH3OH masers. At the same time, we can also obtain an estimate of the systemic velocity in the plane of the sky of the disk+star system. Methods. We used the Very Long Baseline Array and the European VLBI Network to observe the 22.2 GHz H2O and 6.7 GHz CH3OH masers in IRAS20126+4104 at a number of epochs suitably distributed in time. The absolute positions of the maser features were established with respect to reference quasars, which allowed us to derive absolute proper motions. Results. From the parallax of the H2O masers we obtain a distance of 1.64 ± 0.05 kpc, which is very similar to the value adopted so far in the literature (1.7 kpc) and confirms that IRAS20126+4104 is a high-mass (proto)star. From the CH3OH masers we derive the component in the plane of the sky of the systemic velocity of the disk+star system (−16 km s −1 in right-ascension and +7.6 km s −1 in declination). Accurate knowledge of the distance and systemic velocity allows us to improve on the model fit to the H2O maser jet presented in a previous study. Finally, we identify two groups of CH3OH maser features, one undergoing rotation in the disk and possibly distributed along a narrow ring centered on the star, the other characterised by relative proper motions indicating that the features are moving away from the disk, perpendicular to it. We speculate that the latter group might be tracing the disk material marginally entrained by the jet. Conclusions. VLBImulti-epoch observations with phase referencing are confirmed to be an excellent tool for distance determinations and for the investigation of the structure and 3D velocity field within a few 100 AU from newly born high-mass stars.

The multicomponent nature of the Vela pulsar nonthermal X-ray spectrum

We report on our analysis of a 274 ks observation of the Vela pulsar with the Rossi X-Ray Timing Explorer (RXTE). The double-peaked, pulsed emission at 2-30 keV, which we had previously detected during a 93 ks observation, is confirmed with much-improved statistics. There is now clear evidence, in both the spectrum and the light curve, that the emission in the RXTE band is a blend of two separate nonthermal components. The spectrum of the harder component connects smoothly with the OSSE, COMPTEL, and EGRET spectra, and the peaks in the light curve are in phase coincidence with those of the high-energy light curve. The spectrum of the softer component is consistent with an extrapolation to the pulsed optical flux, and the second RXTE pulse is in phase coincidence with the second optical peak. In addition, we see a peak in the 2-8 keV RXTE pulse profile at the radio phase.

High‐resolution observations of 6.7‐GHz methanol masers with the Long Baseline Array

We have used the Australian Long Baseline Array (LBA) to produce milliarcsecond images of five sites of methanol (CH3OH) maser emission at 6.7 GHz. These are all sites that have linear morphologies at arcsecond resolutions, which have been hypothesized to be due to the masers forming in edge-on circumstellar discs. We find that a simple disc model cannot explain the observations. We discuss various alternatives, and suggest a new model that explains how linear velocity gradients can be produced in methanol masers that arise in planar shocks propagating nearly perpendicular to the line of sight.

VLBI Observations of Southern Hemisphere ICRF Sources. I

We present 8.4 GHz very long baseline interferometry (VLBI) observations of 69 southern hemisphere extragalactic sources in the International Celestial Reference Frame. These are the first in a series of observations intended to image all such sources at milliarcsecond resolution in order to determine their continued suitability for reference-frame use based on intrinsic structure. We use the resultant images to calculate a core fraction, that is, the ratio of core flux density to total flux density, for all observed sources. The resulting distribution, with a mean value of 0.83, suggests that most sources are relatively compact. However, just over half the observed sources show significant extended emission in the form of multiple compact components. These sources are probably poorly suited for high-accuracy reference-frame use unless intrinsic structure and potential variability can be taken into account. Our observations represent the first large, comprehensive VLBI imaging survey in the southern hemisphere, significantly extending the existing limited VLBI surveys and, along with some well-known objects, containing many sources that have never been imaged at milliarcsecond resolution. The overlap with Very Long Baseline Array images of sources between 0° and -35° declination helps determine the limits to imaging with the southern hemisphere–accessible telescopes.

The Commensal Real-Time ASKAP Fast-Transients (CRAFT) Survey

We are developing a purely commensal survey experiment for fast (<5 s) transient radio sources. Short-timescale transients are associated with the most energetic and brightest single events in the Universe. Our objective is to cover the enormous volume of transients parameter space made available by ASKAP, with an unprecedented combination of sensitivity and field of view. Fast timescale transients open new vistas on the physics of high brightness temperature emission, extreme states of matter and the physics of strong gravitational fields. In addition, the detection of extragalactic objects affords us an entirely new and extremely sensitive probe on the huge reservoir of baryons present in the IGM. We outline here our approach to the considerable challenge involved in detecting fast transients, particularly the development of hardware fast enough to dedisperse and search the ASKAP data stream at or near real-time rates. Through CRAFT, ASKAP will provide the testbed of many of the key technologies and survey modes proposed for high time resolution science with the SKA.

The radio nebula surrounding the Vela pulsar

We have discovered that the radio nebula surrounding the Vela pulsar covers a much wider extent than previously reported, with two lobes to the North and South of the pulsar. Indications of this object have been reported previously, but its symmetric morphology around the pulsar and other details had not been identified as they were hidden due to poor sensitivity to low spatial frequencies. The structure is highly polarised and the polarisation vectors, once corrected for Faraday rotation, reveal symmetry with respect to the spin axis of the pulsar. The X-ray emission found by Chandra lies at the centre of this structure, in a region which has no detectable excess of radio emission. We estimate total fluxes and regional fluxes from the Northern and Southern lobes, plus the X-ray region at four radio frequencies; 1.4, 2.4, 5 and 8.5 GHz. We present the corresponding images in both the total and polarised intensities, as well as those showing the derotated linear polarisation vectors.

HIGH-PRECISION ASTROMETRIC MILLIMETER VERY LONG BASELINE INTERFEROMETRY USING A NEW METHOD FOR ATMOSPHERIC CALIBRATION

We describe a new method which achieves high-precision very long baseline interferometry (VLBI) astrometry in observations at millimeter (mm) wavelengths. It combines fast frequency-switching observations, to correct for the dominant non-dispersive tropospheric fluctuations, with slow source-switching observations, for the remaining ionospheric dispersive terms. We call this method source-frequency phase referencing. Provided that the switching cycles match the properties of the propagation media, one can recover the source astrometry. We present an analytic description of the two-step calibration strategy, along with an error analysis to characterize its performance. Also, we provide observational demonstrations of a successful application with observations using the Very Long Baseline Array at 86 GHz of the pairs of sources 3C274 and 3C273 and 1308+326 and 1308+328 under various conditions. We conclude that this method is widely applicable to mm-VLBI observations of many target sources, and unique in providing bona fide astrometrically registered images and high-precision relative astrometric measurements in mm-VLBI using existing and newly built instruments, including space VLBI.