THE OPTICAL COUNTERPART TO THE GAMMA-RAY BURST GRB970508

Understanding the nature of the gamma-ray burst phenomenon is one of t he outstanding problems of modern astrophysics, The identification of counterparts at optical wavelengths is considered a crucial factor for determining the origin of these events, Here we report the detection and temporal properties of a variable optical source, which has been i dentified(1,2) as the counterpart of the X-ray transient associated wi th the gamma-ray burst GRB970508 (ref. 3). The initial optical images were obtained only 5.8 hours after the initial gamma-ray burst, after which the optical source was observed to brighten over the next two da ys before declining in luminosity with a t(-1) power law, The decline in brightness follows a form predicted by many relativistic fireball m odels(4-7) for gamma-ray bursts, although the initial rise does not ap pear to be compatible with the simplest of these models, The observed fluence of the source at visible wavelengths over the period spanned b y our observations is greater than or equal to 4.6 x 10(-8) erg cm(-2) , about 3% of the fluence of the gamma-ray burst itself.