Optical light curve and cooling break of GRB 050502A

We present light curves of the afterglow of GRB 050502A, including very early data at ttGRB <60 s. The light curve is composed of unfiltered ROTSE-IIIb optical observations from 44 s to 6 hr postburst, R-band MDM observations from 1.6 to 8.4 hr postburst, and PAIRITEL JHKs observations from 0.6 to 2.6 hr postburst. The optical lightcurveisfitbyabrokenpowerlaw,wheretsteepensfrom � ¼� 1:13 � 0:02to � 1:44 � 0:02at � 5700s.This steepening is consistent with the evolution expected for the passage of the cooling frequencyc through the optical band. Even in our earliest observation at 44 s postburst, there is no evidence that the optical flux is brighter than a backward extrapolation of the later power law would suggest. The observed decay indices and spectral index are consistentwitheitheranISMorawindfireballmodel,butslightlyfavortheISMinterpretation.Theexpectedspectral index in the ISM interpretation is consistent within 1 � with the observed spectral index � ¼� 0:8 � 0:1; the wind interpretation would imply a spectral index slightly (� 2 � ) shallower than observed. A small amount of dust extinc- tion at the source redshift could steepen an intrinsic spectrum sufficiently to account for the observed value of � .I n this picture, the early optical decay, with the peak at or below 4:7 ;10 14 Hz at 44 s, requires very small electron and magnetic energy partitions from the fireball. Subject headingg gamma rays: bursts