Optical follow-up of GRB 970508

T. J. Galama, P. J. Groot, J. van Paradijs, C. Kouveliotou, R. G. Strom, R. A. M. J. Wijers, N. Tanvir, J. Bloom, M. Centurion, J. Telting, R. G. M. Rutten, P. Smith, C. Mackey, S. Smartt, C. Benn, J. Heise, J. in 't Zand

Published 1998-02-12Version 1

We report on the results of optical follow-up observations of the counterpart of GRB 970508, starting 7 hours after the event. Multi-color 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 $\sim$ 4 months after the burst. The light curve after the maximum follows a decline that can be fitted with a power law with exponent $\alpha$ = --1.141 $\pm$ 0.014. Deviations from a smooth power law decay are moderate (r.m.s. = 0.15 magnitude). We find no flattening of the light curve at late times. The optical afterglow fluence is a significant fraction, $\sim$ 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).