R. N. Manchester, L. Staveley-Smith, A. K. Tzioumis, M. J. Kesteven, J. E. Reynolds (ATNF); B. M. Gaensler (Centre for Space Research, Massachusetts, USA); V. C. Wheaton (ATNF/Sydney University): N. S. Bizunok (Centre for Space Research, Massachusetts, USA/Boston University, USA)

One of the principal scientific justifications for the 12-mm system on the Compact Array, part of the 1997 MNRF upgrade project, was to obtain higher resolution images of continuum sources, and of the radio remnant of SN 1987A in particular. SN 1987A, first observed on 23 February 1987, was (and remains) the brightest supernova observed for 400 years. It occurred in the Large Magellanic Cloud, close to the giant nebula 30 Doradus. Optically, the supernova was visible with large telescopes for more than a decade, but the initial burst of radio emission, the radio supernova, faded in just a few days. Following this, no radio emission was detected for more than three years. In July 1990, radio emission was again detected with the Molonglo Synthesis Telescope and (for the first time) with the Compact Array. Since then, monitoring observations at both Molonglo and the Compact Array have shown that the emission has increased more or less steadily (Figure 1). This increasing emission signals the birth of a supernova remnant, the first time such an event has been observed.

Observations with the 6-km Compact Array at 3 cm have a diffraction-limited beam size of slightly less than 1 arcsecond. Currently, the diameter of the remnant, SNR 1987A, is about 1.6 arcseconds, so it is barely resolved. We have been able to apply super-resolution techniques to the data to obtain apparently reliable images with a resolution of 0.5 arcsecond. These show a shell structure, with enhanced emission on the east and west sides, corresponding to the major axis of the projected central ring observed in optical recombination lines and lying within this ring.

At the completion of the MNRF upgrade, expected in the first half of 2003, all six Compact Array antennas will be equipped with 12-mm systems. It is likely that this system will provide the highest-resolution high-dynamic-range continuum images that are possible with the Compact Array. The minimum beam size will be about 0.3 arcsecond, significantly better than the super-resolved 3-cm beam size. It is possible that super-resolution can be applied to the 12-mm images as well, resulting in a resolution comparable to that of the Hubble Space Telescope and exceeding that of the X-ray satellite observatory Chandra. This will be of enormous benefit in improving our understanding of the astrophysics of this system.

In October 2001, three antennas (CA02, 03 and 04) were equipped with interim 12-mm receivers and phase-stable local oscillator systems. Twelve hours of data on SNR 1987A were obtained at two frequencies, 16.96 and 18.88 GHz, within the 12-mm band. The systems were dual polarization and had a bandwidth of 128 MHz at each frequency. Phase stability was excellent for all but the last 2.5 hours. The maximum baseline was 1.1 km, giving a beam size of approximately 2.8 by 1.8 arcseconds. The remnant was clearly detected at both frequencies, with an integrated flux density of 20 milliJansky at the lower frequency and 18 milliJansky at the higher frequency. These values are slightly higher than those predicted from the cm-band spectrum, but consistent with the prediction within the uncertainties. Figure 2 shows the image obtained by combining data from the two frequencies. The remnant was barely resolved by these observations, giving an image which is similar to the diffraction-limited image at 3 cm.

This is the first image of SNR 1987A at 12 mm. The results were reported in an IAU Circular, the first publication resulting from the MNRF upgrade. They make it clear that the completed 12-mm system will be able to produce superb images of the supernova remnant.

back to highlights page