Mark Hendrik Wieringa

Radio emission from the unusual supernova 1998bw and its association with the gamma-ray burst of 25 April 1998

Data accumulated over the past year strongly favour the idea that γ-ray bursts lie at cosmological distances, although the nature of the power source remains unclear. Here we report radio observations of the supernova SN1998bw, which exploded at about the same time, and in about the same direction, as the γ-ray burst GRB980425. At its peak, the supernova was unusually luminous at radio wavelengths. A simple interpretation of the data requires that the source expanded with an apparent velocity of at least twice the speed of light, indicating that the supernova was accompanied by a shock wave moving at relativistic speeds (the ejects of supernovae are typically characterized by non-relativistic velocities). The energy of the shock is at least 1049 erg, with an inferred ejecta mass of 10−5 solar masses, and we suggest that the early phase of this shock wave produced the burst of γ-rays. Although in general the properties of supernovae are very different from those of γ-ray bursts, we argue that this unusual supernova establishes a second class of γ-ray burst, which is distinctly different from the cosmological kind.

The Australia Telescope Compact Array Broad-band Backend: description and first results

Here we describe the Compact Array Broadband Backend (CABB) and present first results obtained with the upgraded Australia Telescope Compact Array (ATCA). The 16-fold increase in observing bandwidth, from 2×128 MHz to 2×2048 MHz, high bit sampling, and addition of 16 zoom windows (each divided into a further 2048 channels) provide major improvements for all ATCA observations. The benefits of the new system are: (1) hugely increased radio continuum and polarization sensitivity as well as image fidelity, (2) substantially improved capability to search for and map emission and absorption lines over large velocity ranges, (3) simultaneous multi-line and continuum observations, (4) increased sensitivity, survey speed and dynamic range due to high-bit sampling, and (5) high velocity resolution, while maintaining full polarization output. The new CABB system encourages all observers to make use of both spectral line and continuum data to achieve their full potential. Given the dramatic increase of the ATCA capabilities in all bands (ranging from 1.1 to 105 GHz) CABB enables scientific projects that were not feasible before the upgrade, such as simultaneous observations of multiple spectral lines, on-the-fly mapping, fast follow-up of radio transients (e.g., the radio afterglow of new supernovae) and maser observations at high velocity resolution and full polarization. The first science results presented here include wide-band spectra, high dynamic-range images, and polarization measurements, highlighting the increased capability and discovery potential of the ATCA.

Radio Polarization from the Inner Galaxy at Arcminute Resolution

The Southern Galactic Plane Survey (SGPS) is an H I and 1.4 GHz continuum survey of the fourth quadrant of the Galaxy at arcminute resolution. We present here results on linearly polarized continuum emission from an initial 28 deg2 test region for the SGPS, consisting of 190 mosaicked pointings of the Australia Telescope Compact Array and covering the range 325.5d<l<332.5d, -0.5d<b<3.5d. Complicated extended structure is seen in linear polarization throughout the test region, almost all of which has no correlation with total intensity. We interpret the brightest regions of polarized emission as representing intrinsic structure in extended polarization, most likely originating in the Crux spiral arm at a distance of 3.5 kpc; fainter polarized structure is imposed by Faraday rotation in foreground material. Two large areas in the field are devoid of polarization. We argue that these voids are produced by foreground H II regions in which the magnetic field is disordered on scales of ~0.1-0.2 pc. We also identify a depolarized halo around the H II region RCW 94, which we suggest results from the interaction of the H II region with a surrounding molecular cloud.

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.

The Southern Galactic Plane Survey: the test region

The Southern Galactic Plane Survey (SGPS) is a project to image the H I line emission and 1.4 GHz continuum in the fourth quadrant of the Milky Way at high resolution using the Australia Telescope Compact Array and the Parkes Radio Telescope. This paper describes the survey details and goals, presents 21 cm continuum data, and discusses H I absorption and emission characteristics of the SGPS test region (3255 ≤ l ≤ 3335; -05 ≤ b ≤ +35). We explore the effects of massive stars on the interstellar medium through a study of H I shells and the H I environments of H II regions and supernova remnants (SNRs). We find an H I shell surrounding the H II region RCW 94, which indicates that the region is embedded in a molecular cloud. We give lower limits for the kinematic distances to SNRs G327.4+0.4 and G330.2+1.0 of 4.3 and 4.9 kpc, respectively. We find evidence of interaction with the surrounding H I for both of these remnants. We also present images of a possible new SNR G328.6-0.0. Additionally, we have discovered two small H I shells with no counterparts in continuum emission.

A Complete Catalog of Radio Afterglows: The First Five Years

We present a catalog of 75 gamma-ray bursts (GRBs) that occurred between 1997 and 2001 and for which accurate positions were obtained by the BeppoSAX satellite, the Interplanetary Network, the Rossi X-Ray Timing Experiment satellite, and the High Energy Transient Experiment satellite. As a result of these localizations, we carried out sensitive searches for radio afterglows using the Very Large Array and the Australia Telescope Compact Array, at a frequency of 5 or 8.5 GHz. Subsequent afterglow detections were followed up using several facilities at centimeter, millimeter, and submillimeter wavelengths. The final catalog has 25 afterglow detections and consists of 1521 flux density measurements (or upper limits) at frequencies between 1.4 and 650 GHz. We argue that our search strategy is capable of identifying radio afterglows and distinguishing them from other extragalactic variables whose areal density we derive to be 1.6 × 10-3 arcmin-2. In addition to complete radio light curves for all cosmological GRBs, the catalog also contains the entire data set for SN 1998bw, an unusual Type Ib/c supernova coincident with GRB 980425.

GRB 011121: A Massive Star Progenitor

Of the cosmological gamma-ray bursts, GRB 011121 has the lowest redshift, z = 0.36. More importantly, the multicolor excess in the afterglow detected in the Hubble Space Telescope (HST) light curves is compelling observational evidence of an underlying supernova. Here we present near-infrared and radio observations of the afterglow, and from our comprehensive afterglow modeling, we find evidence favoring a wind-fed circumburst medium. Lacking X-ray data, we are unable to conclusively measure the mass-loss rate, , but obtain an estimate, ~ 2 × 10-7/vw3 M☉ yr-1, where vw3 is the speed of the wind from the progenitor in units of 103 km s-1. This is similar to that inferred for the progenitor of the Type Ibc supernova SN 1998bw that has been associated with the peculiar burst GRB 980425. Our data, taken in conjunction with the HST results of Bloom et al., provide a consistent picture: the long-duration GRB 011121 had a massive star progenitor that exploded as a supernova at about the same time as the gamma-ray burst event. Finally, we note that the gamma-ray profile of GRB 011121 is similar to that of GRB 980425.

The ATESP radio survey - III. Source counts

This paper is part of a series reporting the results of the ATESP radio survey obtained at 1.4 GHz with the Australia Telescope Compact Array. The survey consists of 16 radio mosaics with ∼

The ATESP radio survey - IV. Optical identifications and spectroscopy in the EIS-A region

8\arcsec \times 14\arcsec

Long-term spectral and timing properties of the soft gamma-ray repeater SGR 1833−0832 and detection of extended X-ray emission around the radio pulsar PSR B1830−08

resolution and uniform sensitivity (