PHOTOMETRY AND SPECTROSCOPY OF THE GRB-970508 OPTICAL COUNTERPART

An optical transient within the error box of the gamma ray burst GRB 9 70508 was imaged 4 hours after the event. It displayed a strong ultrav iolet excess, and reached maximum brightness 2 days later. The optical spectra did not show any emission lines, and no variations on time sc ales of minutes were observed for 1 hour during the decline phase. Acc ording to the fireball and afterglow models, the intensity should rise monotonically before the observed optical maximum, but the data indic ate that another physical mechanism may be responsible for the constan t phase seen during the first hours after the burst.