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~t^-α, where α=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 α=1.0^{+ 0.2}_−0.5 with occasional fluctuations superposed. These two decay light curves are remarkably similar in form to that predicted by cosmic-fireball models.