A Systematic Search for Supernova Light in Gamma-Ray Burst Afterglows
Abstract
We systematically reanalyzed all Gamma-Ray Burst (GRB) afterglow data published through the end of 2001, in an attempt to detect the predicted supernova light component. We fit the observed photometric light curves as the sum of an afterglow, an underlying host galaxy, and a supernova component. The latter is modelled using published UBVRI light curves of SN 1998bw as a template. The total sample of afterglows with established redshifts contains 17 bursts (GRB 970228 - GRB 011211). For six of these GRBs a weak supernova excess (scaled to SN 1998bw) was found. Among them are five bursts with redshift less than ~0.7. For further five bursts with z<1.5 upper limits were established. The general lack of a detection of a supernova component at these larger redshifts can be explained with selection effects. These results support the notion that all afterglows of long-duration GRBs contain light from an associated supernova. The cut off date of our sample (end of 2001) was chosen to ensure that all GRBs had published follow-up observations. Since that date, ~10 new afterglows with redshifts have been established. All but two of these were at redshifts above 0.7, and again for both nearby events (GRB 020405 and GRB 030329) a supernova component was established. This is consistent with the statistical inferences from the sample of earlier long-duration GRBs and leads us to conclude that the current world sample of GRB afterglow measurements provides strong statistical support for the link between (long duration) GRBs and the final stages of massive star evolution. While so far only one event (GRB 030329) allowed a direct spectroscopic confirmation of this link, the larger photometric sample discussed here supports this idea by statistical means.Keywords
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