S. Ryder (Anglo-Australian Observatory); R. Subrahmanyan (ATNF); E. Sadler (University of Sydney); K. Weiler (Naval Research Laboratory, USA)

On the evening of 10 December 2001, the Rev. Robert Evans was conducting his regular supernova patrol from his backyard in the Blue Mountains west of Sydney, when he came across an interloper in the nearby late-type spiral galaxy NGC 7424. In an era when the vast majority of supernova discoveries are made by robotic telescopes, or as part of the high-redshift supernova search programs, Evans trained eye, photographic memory, and trusty 12-inch telescope have allowed him to maintain a proud record of supernova discoveries. Supernova 2001ig was his 39th discovery, and the 241st supernova discovery of 2001.

Since its commissioning, the Compact Array has played a leading role in the monitoring of several supernovae at radio wavelengths, most notably SN 1987A and SN 1978K. Radio emission from Type II supernovae is typically not detected (or even searched for) until weeks, months, or even years after the outburst. On the evening of Saturday 15 December 2001, we made use of six hours of unallocated time on the Compact Array to make images of SN 2001ig at frequencies of 8.6, 4.8, 2.5 and 1.4 GHz. Somewhat to our surprise, there was a positive detection at the highest frequencies. On New Year s Eve, it was detected at 18.8 GHz using the prototype 12-mm receiver system on three Compact Array antennas.

Figure 1 shows an image of the field at 4.8 GHz from Compact Array observations taken on 17 February 2002. SN 2001ig is the upper of the two bright sources shown as contours; the source to the southwest is an unrelated background source. Figure 2 shows the results from our Compact Array monitoring program; for clarity, only the 8.6 and 4.8 GHz data are shown. The radio light curve of a supernova can be divided into three phases a rapid turn-on with a steep spectral index, due in large part to a decrease in the line-of-sight absorption; a peak in the flux density which is first seen at the higher frequencies; a more gradual decline in an optically-thin phase. By March 2002, SN 2001ig had begun to fade at 5 and 8 GHz, but then abruptly doubled in flux and remained that way for three weeks, before resuming its decline. In late August, it again briefly halted its decline. Such dramatic excursions from a smooth decline are rare, and generally thought to be associated with the interaction of the blast ejecta with a dense and rather inhomegenous circumstellar medium.

SN 2001ig is one of only a half dozen or so known examples of a Type IIb supernova, in which the hydrogen lines seen in their early optical spectra soon fade away, suggesting that much of their outer layers were shed before the star exploded. Two possible mechanisms for this, which could also account for the bumps in the radio light curve, are thermal pulses due to carbon/helium flashes in the core of a 5-10 solar mass asymptotic giant branch progenitor star, occurring on periodic time scales of around 500 years, or a modulation of a red supergiant progenitor wind due to eccentric orbital motion about a massive binary companion, leading to a pinwheel-like clumping in the circumstellar medium. Efforts are now underway to model the observed behaviour in the context of these scenarios, while Compact Array monitoring of SN 2001ig will continue throughout 2003.

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