The ATCA captures a "baby" supernova

On the evening of 10 December 2001, the Reverand 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 (D=12.5 Mpc) 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 a trusty 0.31-m (12") telescope have allowed him to maintain a proud record of supernova discoveries. Supernova 2001ig was his 39th discovery, and the 241st of 2001.

Figure 1: Contours of 6-cm (4.8-GHz) radio emission observed with the ATCA on January 15 2002, overlaid on a blue light image of the outskirts of NGC 7424 from the Digitised Sky Survey. SN 2001ig is the upper-left of the two sources detected, and at its peak was comparable in brightness (~15 mJy) to the adjacent source.

Since its commissioning, the Australia Telescope Compact Array has played a leading role in the monitoring of several supernovae at radio wavelengths, most notably SN 1987A (Staveley-Smith et al. 1992) and SN 1978K (Ryder et al. 1993; Schlegel et al. 1999). Radio emission from these objects is typically not detected (or even searched for) until weeks, months, or even years after the outburst. A group led by Kurt Weiler at NRL has over the years been very successful at monitoring several historical and newly-discovered supernovae using the VLA ( see ).

Shortly after the discovery of SN2001ig, a request came from Kurt Weiler for the ATCA to search for a possible radio counterpart. With the VLA in its most compact D-configuration, the southerly declination of NGC 7424 (Dec = -41°) led to substantial shadowing of antennas and poor beam-shape, making the ATCA much better suited to carry out this particular observation. On the evening of Saturday December 15, while most of the Compact Array staff were off in Narrabri for their staff Christmas party, the rostered Duty Astronomer Stuart Ryder, together with Ravi Subrahmanyan, made use of six hours of unallocated time on the ATCA to make images at 8.64, 4.79, 2.50, and 1.38 GHz. Much to their surprise and delight, there was a positive detection of ~2 mJy at 8.64 GHz, and a marginal detection at 4.79 GHz. This result was reported in IAU Circular No. 7777, and since then, the development of SN 2001ig has been monitored at intervals of four to five days, using mostly small blocks of unallocated time on the ATCA. Figure 1 shows an image of the field at 4.8 GHz from January 2002, overlaid on a B-band optical image from the Digitised Sky Survey. SN 2001ig is the northeast of the two bright sources; the source to the southwest is assumed to be an unrelated background source, which has proven to be extremely useful as a position and flux reference, particularly on days of poor phase stability at the higher frequencies.

Perhaps most exciting of all was a detection on New Years Eve 2001 with the new 12 mm system on only three antennas, making SN2001ig only the second supernova ever to be detected at millimetre wavelengths. As the VLA antennas are progressively reconfigured into their most extended A-configuration, they have been contributing more of the data at higher frequencies, while the ATCA concentrates on the lower frequencies. In due course, observations with the Molonglo Observatory Synthesis Telescope (MOST) should give us complete coverage spanning a factor of 30 in wavelength.

Figure 2 shows the results of our joint ATCA/VLA monitoring program to date. The radio "light curve" of a supernova can be divided into three phases — a rapid turn-on with a steep spectral index (alpha = -2 or steeper, where S ~ nalpha), due in large part to a decrease in the line-of-sight absorption; followed by a peak in the flux density, firstly at the higher frequencies; then a more gradual decline in the optically-thin phase. The curves fitted to the data in Figure 2 are a modified version of the minishell model of Chevalier (1982) as described in Weiler et al. (1990) and Montes et al. (1997). The flux variation with time t and frequency n is given by:

Of particular interest in recent studies of radio SNe is what produces the intervening absorption in the first place: is it free-free absorption in a circumstellar medium of synchrotron emission arising close to the supernova shock wave? Or does synchrotron self-absorption (SSA) have a role to play as well, as demonstrated in the case of SN 1987A (Storey & Manchester 1987; Chevalier & Dwarkadas 1995) and of SN 1993J (Fransson & Björnsson 1998)? By obtaining observations of the rising part of the radio light curve, particularly at the lower frequencies, it is possible to make an assessment of the relative contributions. In the absence of SSA, we can expect the flux at 1.4 GHz to peak some 75 days after the explosion (Figure 2); otherwise, SSA will act to postpone this peak by perhaps a couple of months. Continued monitoring through the rest of the ATCA 02JANT term will be fundamental to answering this question. Studies of the absorption mechanism can be used to place constraints on the magnetic field strength, expansion velocity, and pre-supernova mass-loss rate. Ongoing monitoring in the decline phase may even reveal evidence of episodic variations in the supernova progenitor mass-loss rate, as seen in SN 1979C (Weiler et al. 1992).

Figure 2: Radio "light curve" of SN 2001ig up to January 15 2002. The open black circles are a mix of 18.8-GHz observations from the ATCA, and 22.5-GHz observations from the VLA. The physical nature of the fitted models, and the meaning of the various parameters, are described in the text, as well as in Montes et al. (1997).

Both the optical and radio behaviour of SN 2001ig bear a marked similarity to another nearby Type IIb supernova, SN 1993J in M81. On account of its proximity (3.6 Mpc) and brightness, SN 1993J has been intensively studied with global VLBI networks, allowing the physical expansion of the remnant to be observed (Marcaide et al. 1997). Although SN 2001ig is about 4´ more distant than SN 1993J, its 4.8 GHz peak luminosity is ~3´ greater, so there is every chance we will be able to resolve the remnant in the months and years ahead using the Australian LBA, perhaps in conjunction with the southern-most antennas of the VLBA. ATCA observations of SN 2001ig have already given us a rare insight into the early development of a young supernova into a supernova remnant, and the story is only just beginning.

Stuart Ryder (Anglo-Australian Observatory), Kelly Kranz (Anglo-Australian Observatory), Ravi Subrahmanyan (ATNF), Elaine Sadler (U. Sydney), and Kurt Weiler (Naval Research Laboratory)

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