Ravi Subrahmanyan

The properties of extragalactic radio sources selected at 20 GHz

We present some first results on the variability, polarization and general properties of radio sources selected at 20 GHz, the highest frequency at which a sensitive radio survey has been carried out over a large area of sky. Sources with flux densities above 100 mJy in the Australia

Modulations in the radio light curve of the Type IIb supernova 2001ig: evidence for a Wolf–Rayet binary progenitor?

We describe the radio evolution of supernova (SN) 2001ig in NGC 7424, from 700 d of multifrequency monitoring with the Australia Telescope Compact Array (ATCA) and the Very Large Array (VLA). We find that deviations of the radio light curves at each frequency from the standard ‘minishell’ model are consistent with density modulations in the circumstellar medium (CSM), which seem to recur with a period near 150 d. One possibility is that these are due to enhanced mass loss from thermal pulses in an asymptotic giant branch star progenitor. A more likely scenario, however, is that the progenitor was a Wolf–Rayet (WR) star, whose stellar wind collided with that from a massive hot companion on an eccentric 100-d orbit, leading to a regular build-up of CSM material on the required time and spatial scales. Recent observations of ‘dusty pinwheels’ in WR binary systems lend credibility to this model. Since such binary systems are also thought to provide the necessary conditions for envelope stripping which would cause the WR star to appear as a Type Ib/c SN event rather than a Type II, these radio observations of SN 2001ig may provide the key to linking Type Ib/c SNe to Type IIb events, and even to some types of gamma-ray bursts.

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.

A Complete Sample of Megaparsec-sized Double Radio Sources from the Sydney University Molonglo Sky Survey

We present a complete sample of megaparsec-size double radio sources compiled from the Sydney University Molonglo Sky Survey (SUMSS). Almost complete redshift information has been obtained for the sample. The sample has the following defining criteria: Galactic latitude |b| > 12. 5, declination δ < −50 and angular size > 5. All the sources have projected linear size larger than 0.7 Mpc (assuming Ho = 71 km s −1 Mpc). The sample is chosen from a region of the sky covering 2100 square degrees. In this paper, we present 843-MHz radio images of the extended radio morphologies made using the Molonglo Observatory Synthesis Telescope (MOST), higher resolution radio observations of any compact radio structures using the Australia Telescope Compact Array (ATCA), and low resolution optical spectra of the host galaxies from the 2.3-m Australian National University (ANU) telescope at Siding Spring Observatory. The sample presented here is the first in the southern hemisphere and significantly enhances the database of known giant radio sources. The giant radio sources with linear size exceeding 0.7 Mpc have an abundance of (215 Mpc) at the sensitivity of the survey. In the low redshift universe, the survey may be suggesting the possibility that giant radio sources with relict lobes are more numerous than giant sources in which beams from the centre currently energize the lobes. Subject headings: Surveys – samples: galaxies – radio galaxies: continuum – optical spectra CSIRO Australia Telescope National Facility, Locked bag 194, Narrabri NSW 2390, Australia. Lakshmi.Saripalli@csiro.au and Ravi.Subrahmanyan@ csiro.au School of Physics, University of Sydney, NSW 2006, Australia. rwh@physics.usyd.edu.au MIT Kavli Institute, 77 Massachusetts Avenue, Cambridge MA 02139. eboyce@mit.edu

A case for renewed activity in the giant radio galaxy J0116-473

We present Australia Telescope Compact Array (ATCA) radio observations of the giant radio galaxy J0116-473 at 12 and 22 cm wavelengths in total intensity and polarization. The images clearly reveal a bright inner-double structure within more extended edge-brightened lobe emission. The lack of hot spots at the ends of the outer lobes, the strong core, and the inner-double structure with its edge-brightened morphology lead us to suggest that this giant radio galaxy is undergoing renewed nuclear activity: J0116-473 appears to be a striking example of a radio galaxy in which a young double source is evolving within older lobe material. We also report the detection of a megaparsec-long linear feature that is oriented perpendicular to the radio axis and has a high fractional polarization.

H i 21 cm absorption at z ∼ 2.347 towards PKS B0438-436

We report the detection of redshifted H i 21-cm absorption in the z ~ 2.347 damped Lyman absorber (DLA) towards PKS B0438-436 with the Green Bank Telescope. This is the second-highest redshift at which 21-cm absorption has been detected in a DLA. The absorption extends over ˜60 km s −1 and has two distinct components, at z= 2.347 477(12) and z= 2.347 869(20). A similar velocity structure is seen in optical metal lines, although the peak absorption here is offset by ~11 km s −1 from the peak in the 21-cm line. We obtain a high spin temperature T s ~ (886 ±248) × (f/0.58) K, using a covering factor estimated from 2.3-GHz very long baseline interferometry data. However, the current data cannot rule out a low spin temperature. The non-detection of CO 3-2 absorption places the upper limit N CO 15 (T x /10) cm −2 on the CO column density.

Extragalactic sources towards the central region of the Galaxy

We have observed a sample of 64 small-diameter sources towards the central -6° < l < 6°, -2° < b < 2° of the Galaxy with the aim of studying the Faraday rotation measure near the Galactic Centre region. All the sources were observed at 6- and 3.6-cm wavelengths using the ATCA and the VLA. 59 of these sources are inferred to be extragalactic. The observations presented here constitute the first systematic study of the radio polarization properties of the background sources towards this direction and increase the number of known extragalactic radio sources in this part of the sky by almost an order of magnitude. Based on the morphology, spectral indices and lack of polarized emission, we identify four Galactic H II regions in the sample.

Renewed Activity in the Radio Galaxy PKS B1545–321: Twin Edge-brightened Beams within Diffuse Radio Lobes

Australia Telescope Compact Array images of the giant radio galaxy PKS B1545-321 show a pair of oppositely directed beams emerging from a radio core and ending in bright components that are symmetrically located on either side. These inner beams are embedded within edge-brightened outer lobes of lower surface brightness, and the bright ends of the inner beams are well recessed from the ends of the outer lobes. The inner beams and diffuse surrounding lobes share a common central core and radio axis. We propose that the observed inner beams are double lobes that have been created within relic outer lobes as a consequence of a restarting of the central activity; therefore, PKS B1545-321 is a rare opportunity for examining the development of restarted beams within a relic synchrotron plasma cocoon. The inner double representing the new episode has among the highest axial ratios found in typical edge-brightened radio galaxies. The low radio luminosity of the inner double, the narrow and constant transverse extent of its cocoon, and the relatively low brightness of the hot spots at its ends are consistent with the almost ballistic propagation expected for a beam that has a low density contrast and is advancing within a relatively light ambient medium.

The temperature of the warm neutral medium in the Milky Way

We report high-spectral-resolution Australia Telescope Compact Array (ATCA) H I 21-cm observations resulting in the detection of the warm neutral medium (WNM) of the Galaxy in absorption against two extragalactic radio sources, PKS 1814-637 and PKS 0407-658. The two lines of sight were selected on the basis of the simplicity of their absorption profiles and the strength of the background sources; the high velocity resolution of the spectra then enabled us to estimate the kinetic temperatures of the absorbing gas by fitting multiple Gaussians to the absorption profiles. Four separate WNM components were detected towards the two sources, with peak optical depths tau(max)= (1.0 +/- 0.08) x 10(-2), (1.4 +/- 0.2) x 10(-3), (2.2 +/- 0.5) x 10(-3) and (3.4 +/- 0.5) x 10(-3) and kinetic temperatures T-k = 3127 +/- 300, 3694 +/- 1595, 3500 +/- 1354 and 2165 +/- 608 K, respectively. All four components were thus found to have temperatures in the thermally unstable range 500 T-k <5000 K; this suggests that thermal equilibrium has not been reached throughout the WNM.

PKS B1400−33: An Unusual Radio Relic in a Poor Cluster

We present new arcminute-resolution radio images of the low surface brightness radio source PKS B1400-33, which is located in the poor cluster Abell S753. The observations consist of 330 MHz Very Large Array, 843 MHz Molonglo Observatory Synthesis Telescope, and 1398 and 2378 MHz Australia Telescope Compact Array data. These new images, with higher surface brightness sensitivity than previous observations, reveal that the large-scale structure consists of extended filamentary emission bounded by edge-brightened rims. The source is offset to one side of symmetrically distributed X-ray emission that is centered on the dominant cluster galaxy NGC 5419. PKS B1400-33 is a rare example of a relic in a poor cluster with radio properties unlike those of most relics and halos observed in cluster environments. The diffuse source appears to have had an unusual origin, and we discuss possible mechanisms. We examine whether the source could be reenergized relic radio plasma or a buoyant synchrotron bubble that is a relic of activity in NGC 5419. The more exciting prospect is that the source is relic plasma preserved in the cluster gaseous environment following the chance injection of a radio lobe into the intracluster medium as a result of activity in a galaxy at the periphery of the cluster.