The sub-energetic γ-ray burst GRB 031203 as a cosmic analogue to the nearby GRB 980425

Abstract

Over the six years since the discovery of the γ-ray burst GRB 980425, which was associated 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 γ-rays by three orders of magnitude. Radio calorimetry 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 γ-ray luminosity, 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 γ-ray burst. Such sub-energetic events have faint afterglows. We expect intensive follow-up of faint bursts with smooth γ-ray light curves (common to both GRB 031203 and 980425) to reveal a large population of such events.