1. Caltech Optical Observatories 105-24, California Institute of Technology, Pasadena, California 91125, USA
2. Stanford Linear Accelerator Center, 2575 Sand Hill Road M/S 29, Menlo Park, California 94025, USA
3. National Radio Astronomy Observatory, PO Box 0, Socorro, New Mexico 87801, USA
4. Space Radiation Laboratory 220-47, California Institute of Technology, Pasadena, California 91125, USA
5. Carnegie Observatories, 813 Santa Barbara Street, Pasadena, California 91101, USA
6. Department of Astronomy, University of Michigan, 830 Dennison Bldg, Ann Arbor, Michigan 48109, USA

Correspondence to: A. M. Soderberg¹ Email: ams@astro.caltech.edu

Abstract

Over the six years since the discovery¹ of the -ray burst GRB 980425, which was associated² with the nearby (distance 40 Mpc) supernova 1998bw, astronomers have debated fiercely the nature of this event. Relative to bursts located at cosmological distance (redshift z 1), GRB 980425 was under-luminous in -rays by three orders of magnitude. Radio calorimetry^{3, 4} showed that the explosion was sub-energetic by a factor of 10. Here we report observations of the radio and X-ray afterglow of the recent GRB 031203 (refs 5–7), which has a redshift of z = 0.105. We demonstrate that it too is sub-energetic which, when taken together with the low -ray luminosity⁷, suggests that GRB 031203 is the first cosmic analogue to GRB 980425. We find no evidence that this event was a highly collimated explosion viewed off-axis. Like GRB 980425, GRB 031203 appears to be an intrinsically sub-energetic -ray burst. Such sub-energetic events have faint afterglows. We expect intensive follow-up of faint bursts with smooth -ray light curves^{8, 9} (common to both GRB 031203 and 980425) to reveal a large population of such events.

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