Power-law decays in the optical counterparts of GRB 970228 and GRB 970508

We report on R-C- and K-band observations of the optical counterpart to GRB 970508 with the Fred Lawrence Whipple Observatory (FLWO) 1.2 m telescope. Eleven R-C-band observations were obtained on 1997 May 12, and three on May 14. The counterpart clearly faded between the two nights. On May 12 there was no evidence for variability (<9%) on 10-70 minute timescales based on 11 R-c-band observations. On May 19 a 1 hr observation set a limit on the K magnitude of K > 18.6. Comparison of these data points with those obtained by other authors shows that the decay of the optical counterpart can be well fitted by a power law of the form f similar to t(-alpha), where alpha = 1.22 +/- 0.03 with occasional fluctuations superposed. We note that the decay of the optical counterpart to another burst, GRB 970228, can also be well fitted with a power law with exponent alpha = 1.0(-0.5)(+0.2) with occasional fluctuations superposed. These two decay light curves are remarkably similar in form to that predicted by cosmic-fireball models.