On the Radio–to–X-Ray Light Curves of SN 1998[CLC]bw[/CLC] and GRB 980425

Abstract

The discovery of the peculiar supernova SN 1998bw in the error box of the gamma-ray burst GRB980425 set a precedent for the possibility of the physical association of gamma-ray bursters with supernovae. We predict a late-time optical light curve of SN 1998bw, which is the result of thermal emission from a mildly relativistic ejecta heated by radioactivity. We also calculate radio-to-X-ray light curves of the afterglows that are caused by non-thermal emission from a highly relativistic blast wave, which is inferred from the gamma-ray flux detected in GRB 980425 as well as from the very bright radio emission detected in SN 1998bw. We find that the observed gamma-ray and radio light curves are roughly reproduced by the synchrotron emission from a relativistic fireball with a range of Lorentz factors. The optical flux predicted for the non-thermal emission is well below that of the thermal emission observed as SN 1998bw so that it will not be seen at least for a few years. The model predicts the X-ray flux just above the detection limit of BeppoSAX for the epoch when it was pointed to the field of GRB980425. Therefore, the non-detection of X-ray and optical afterglows is consistent with the model. These results suggest that assuming the physical association between SN 1998bw and GRB980425 does not raise any problem in their theoretical interpretation and that they might correspond to an event similar to the 'hypernova' or 'collapsar', where the collapse of a massive star forms a rotating black hole and a disc of the remnant stellar mantle, eventually producing a relativistic shock to make a gamma-ray burst and a bulk mass ejection that can possibly be seen as an odd supernova such as SN 1998bw.