GRB 970228 Revisited: Evidence for a Supernova in the Light Curve and Late Spectral Energy Distribution of the Afterglow

Although at the time of its discovery, the optical and X-ray afterglow of GRB 970228 appeared to be a ringing endorsement of the previously untried relativistic blastwave model of gamma-ray burst (GRB) afterglows, now that nearly a dozen optical afterglows to GRBs have been observed, the wavering light curve and reddening spectrum of this afterglow make it perhaps the most difficult of the observed afterglows to reconcile with the blastwave model. In this Letter, we argue that this afterglow’s unusual temporal and spectral properties can be attributed to a supernova that overtook the light curve nearly two weeks after the GRB. This is the strongest case yet for a GRB/supernova connection. It strengthens the case that a supernova also dominated the late afterglow of GRB 980326, and possibly, the case that GRB 980425 is related to SN 1998bw. Subject headings: gamma-rays: bursts — supernovae: general