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We report on R_c and K band observations of the optical counterpart to GRB970508 with the Fred Lawrence Whipple Observatory (FLWO) 1.2m telescope. ElevenR_c-band observations were obtained on 1997 May 12, and three on May14. Thecounterpart clearly faded between the two nights. On May 12 there was noevidence for variability (<9%) on 10-70 minute time scales based on 11 R_c-bandobservations. On May 19 a one hour observation set a limit on the K magnitudeof K>18.2. Comparison of these data points with those obtained by other authorsshows that the decay of the optical counterpart can be well fitted by a powerlaw of the form f ~ t**-alpha where alpha = 1.22 +/- 0.03 with occasionalfluctuations superposed. We note that the decay of the optical counterpart toanother burst, GRB 970228, can also be well fitted with a power law withexponent alpha = 1.0/+0.2/-0.5 with occasional fluctuations superposed. Thesetwo decay light curves are remarkably similar in form to that predicted bycosmic-fireball models.Comment: replacement, now accepted by ApJ(lett), 8 pages, 1 figur

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