The unusual afterglow of the gamma-ray burst of 26 March 1998 as evidence for a supernova connection

Cosmic gamma-ray bursts have now been firmly established as one of the most powerful phenomena in the Universe, releasing almost the rest-mass energy of a neutron star within the space of a few seconds (ref. 1). The two most popular models to explain gamma-ray bursts are the coalescence of two compa ct objects such as neutron stars or black holes, or the catastrophic collap se of a massive star in a very energetic supernova-like explosion(2,3). Her e we show that, about three weeks after the gamma-ray burst of 26 March 199 8, the transient optical source associated with the burst brightened to abo ut 60 times the expected flux, based upon an extrapolation of the initial l ight curve. Moreover, the spectrum changed dramatically, with the colour be coming extremely red. We argue that the new source is an underlying superno va. If our hypothesis is true then this provides evidence linking cosmologi cally located gamma-ray bursts with deaths of massive stars.