The late-time light curve of SN 1998bw associated with GRB 980425

We report 139 photometric observations through the B, V, and I filters of the supernova SN 1998bw, an object which is associated with the gamma-ray burst GRB 980425. Detailed light curves of this unique supernova can be compared with theoretical models, so we report here our light curve for 123 days between 1998 June 27 and 1998 October 28. The light curve of SN 1998bw is consistent with those of the Type Ic class. We find that the magnitude versus time relation for this supernova is linear to within 0.05 mag in all colors over the entire duration of our study. Our measured uniform decline rates are 0.0141 +/- 0.0002, 0.0184 +/- 0.0003, and 0.0181 +/- 0.0003 mag per day in the B, V, and I bands. The linear decline and the rate of that decline suggest that the late-time light curve is powered by the radioactive decay of cobalt with some leakage of the gamma rays.