Power-Law Decays in the Optical Counterparts of GRB 970228 and GRB 970508

We report on RC- and K-band observations of the optical counterpart to GRB 970508 with the Fred Lawrence Whipple Observatory (FLWO) 1.2 m telescope. Eleven RC-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 11Rc-band observations. On May 19 a 1 hr observation set a limit on the K magnitude of . Comparison of these data points with those obtained by other authors K 1 18.6 shows that the decay of the optical counterpart can be well fitted by a power law of the form , where 2a f » t with occasional fluctuations superposed. We note that the decay of the optical counterpart to a 5 1.22 5 0.03 another burst, GRB 970228, can also be well fitted with a power law with exponent with occasional 10.2 a 5 1.020.5 fluctuations superposed. These two decay light curves are remarkably similar in form to that predicted by cosmicfireball models. Subject heading: gamma rays: bursts