SCUBA observations of GRB 030329:
The brightest submillimeter afterglow to date

As already discussed in the March 2002 and March 2003 JCMT Newsletters, gamma-ray bursters (GRBs) are currently attracting intense multiwavelength studies to unravel their nature. The study of GRBs has made a significant leap forward in the last few years thanks to the discoveries of transient counterparts and quiescent host galaxies. These have shown that the bursts are due to enormous explosions taking place throughout the early Universe.

The previous Newsletter articles discussed the SCUBA observations of the host galaxies of the bursts. These measurements are usually made in good observing conditions long after any early emission from the burst itself has faded away. Their aim is to study the star formation rate in the host galaxies and to investigate the link between GRBs and massive star formation throughout the Universe.

Of equal interest is to study the early emission from GRBs. This consists of the emission from the initial explosion, and the "afterglow" that comes from the expanding fireball as it sweeps up the surrounding medium. Submillimeter observations of the early afterglows are of interest because this is where the emission peaks in some bursts in the hours to weeks following the burst. By tracking the evolving afterglow emission across the entire spectrum, it is possible to study aspects such as the types of shocks involved, the geometry of the outflow (jet versus spherical), and the geometry of the surrounding medium (uniform versus prior stellar wind).

Starting with GRB 970508, we have been performing Target of Opportunity observations of GRB afterglows using SCUBA. The goal is to observe the source as quickly as possible when the afterglow is bright, and then follow the evolution of the afterglow flux until the source is no longer detected. Unfortunately, these searches can be hampered by SCUBA not being available. The fact that they must be done quickly also means we are not always able to wait for good weather or for the source to be in an optimal location on the sky.

GRB 030329 - the burst on 2003 March 29 - was the brightest burst seen so far by the HETE satellite. The X-ray localization provided by HETE allowed ground-based optical observers to rapidly locate a bright optical transient. The redshift was measured to be 0.1685, or a luminosity distance of 880 Mpc. This is one of the closest GRBs localized to date, which is consistent with the brightness of the afterglow at all wavelengths.

Observations of the optical light curve in the days following the burst found a very complex evolution of bumps and breaks. Most interesting was the late-time appearance of a supernova, SN 2003dh, whose spectrum was similar to the Type Ic "hypernova" SN 1998bw. While the presence of supernovae has been inferred from the light curves and colors of other gamma-ray burst afterglows, this was the first direct, spectroscopic evidence that at least some classical GRBs originate from core-collapse supernovae.

Unfortunately, SCUBA was not on the telescope at the time of the burst. Although it was several days before the first SCUBA observation could be made, and the weather was not good, GRB 030329 was still easily detected as a 30 mJy source at 850 microns (Hoge et al. 2003, GCN 2088). As at other wavelengths, GRB 030329 was by far the brightest SCUBA afterglow detection to date. This source subsequently faded away, leaving no evidence for an underlying dusty host galaxy.

Observations of new bursts are continuing to produce surprises, and there is much left to learn about GRB afterglows and host galaxies. To obtain a complete picture of their nature will require the careful study of many bursts. To this end, our program of Target of Opportunity observations is ongoing. The Swift satellite - due to be launched in 2004 January - should significantly increase the number of bursts that are rapidly localized. Swift will also localize short bursts, whose counterparts have not been studied so far. Thus we should be better able to perform SCUBA observations in good observing conditions on those bursts that appear to be the most interesting.

We would like to thank the Director for supporting the Target of Opportunity GRB program. We acknowledge the sterling effort of the telescope operators who perform these observations on short notice. And we thank (and apologize to) the many observers whose programs we have bumped over the years.

For more details on this program, please see Smith et al. 1999, A&A, 347, 92, Smith et al. 2001, A&A, 380, 81, and references therein.

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