RADIO-EMISSION FROM THE UNUSUAL SUPERNOVA 1998BW AND ITS ASSOCIIALIONWITH THE GAMMA-RAY BURST OF 25 APRIL 1998

Data accumulated over the past year strongly favour the idea that gamm a-ray bursts lie at cosmological distances, although the nature of the power source remains unclear. Here we report radio observations of th e supernova SN1998bw, which exploded at about the same time, and in ab out the same direction, as the gamma-ray burst GRB980425, At its peak, the supernova was unusually luminous at radio wavelengths. A simple i nterpretation of the data requires that the source expanded with an ap parent velocity of at least twice the speed of light, indicating that the supernova was accompanied by a shock wave moving at relativistic s peeds (the ejects of supernovae are typically characterized by non-rel ativistic velocities). The energy of the shock is at least 10(49) erg, with an inferred ejecta mass of 10(-5) solar masses, and we suggest t hat the early phase of this shock wave produced the burst of gamma-ray s, Although In general the properties of supernovae are very different from those of gamma-ray bursts, we argue that this unusual supernova establishes a second class of gamma-ray burst, which Is distinctly dif ferent from the cosmological kind.