Evolution of the Radio Remnant of SN 1987A: 1990 -- 2001

Abstract

The development of the radio remnant of SN 1987A has been followed using the Australia Telescope Compact Array since its first detection in 1990 August. The remnant has been observed at four frequencies, 1.4, 2.4, 4.8 and 8.6 GHz, at intervals of 4 - 6 weeks since the first detection. These data are combined with the 843 MHz data set of Ball et al. (2001) obtained at Molonglo Observatory to study the spectral and temporal variations of the emission. These observations show that the remnant continues to increase in brightness, with a larger rate of increase at recent times. They also show that the radio spectrum is becoming flatter, with the spectral index changing from -0.97 to -0.88 over the 11 years. In addition, at roughly yearly intervals since 1992, the remnant has been imaged at 9 GHz using super-resolution techniques to obtain an effective synthesised beamwidth of about 0".5. The imaging observations confirm the shell-morphology of the radio remnant and show that it continues to expand at ~3000 km/s. The bright regions of radio emission seen on the limb of the shell do not appear to be related to the optical hotspots which have subsequently appeared in surrounding circumstellar material.