Optical Follow-Up of GRB 970508

Abstract

We report on the results of optical follow-up observations of the counterpart of the gamma-ray burst GRB 970508, starting 7 hr after the event. Multicolor U-, B-, V-, R${}_c$-, and I${}_c$-band observations were obtained during the first three consecutive nights. The counterpart was monitored regularly in R${}_c$ until åisebox-0.5ex 4 months after the burst. The light curve after the maximum follows a decline that can be fitted with a power law with exponent α = -1.141 +/- 0.014. Deviations from a smooth power-law decay are moderate (rms = 0.15 mag). We find no flattening of the light curve at late times. The optical afterglow fluence is a significant fraction, i̊sebox-0.5ex 5%, of the GRB fluence. The optical energy distribution can be well represented by a power law, the slope of which changed at the time of the maximum (the spectrum became redder).