On the Radio-to-X-ray light curves of SN 1998bw and GRB 980425

We calculate radio-to-X-ray light curves for afterglows caused by non-thermal emission from a highly relativistic blast wave, which is inferred from the gamma-ray flux detected in GRB 980425 and 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. The optical flux predicted for the non-thermal emission is well below that of the thermal emission observed for 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 nondetection of X-ray and optical afterglows is consistent with the model. The models presented here are consistent with the physical association between SN 1998bw and GRB980425, and lend further support to the idea that this object might correspond to an event similar to the ``hypernova'' or ``collapsar'' -- events in which the collapse of a massive star forms a rotating black hole surrounded by a disk of the remnant stellar mantle.

K.Iwamoto · 1998