Gamma-ray bursts in relation to black-hole formation

The relatively nearby (40 Mpc) peculiar Type Ic supernova 1998bw was found in the error box of the Gamma Ray Burst GRB980425, and its explosion time coincided within less than one day with the time of the burst. The probability that this was a chance coincidence is less than 10(-6), which strongly suggests that the GRB and the supernova are related. The spectrum and lightcurve of this supernova indicate that the exploding star was a carbon-oxygen star of about 10 M-circle dot, and that the collapsing core had a mass of at least 3 M-circle dot (Iwamoto et al. 1998), too large for forming a stable neutron star. This indicates that here most probably a black hole was formed. The large radio emission of the supernova, with a synchrotron spectrum, indicates that here mass ejection took place with mildly relativistic velocities (v > 0.9c), presumably in the form of a jet, which likely was responsible for the GRB. Although this was intrinsically a very weak GRB, all this seems to provide support for the "collapsar"- or "hypernova"-model which links GRBs with the core collapse of a very massive star to a black hole, as was suggested by Woosley (1993) and Paczynski (1998), respectively. The recent discovery that the optical afterglow lightcurves of two "normal" GRBs at cosmological distances also show evidence for underlying supernova lightcurves similar to that of SN1998bw, provides further strong support for such a link. It thus appears that here for the first time we have been witnessing a new type of supernovae, which are the birth events of black holes as the final products of massive star evolution. Further studies will have to reveal what fraction of all GRBs can be connected with such events.