TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4375 SUBJECT: GRB 051221: Discovery of fading optical afterglow DATE: 05/12/21 19:04:50 GMT FROM: Alicia Soderberg at Caltech A. M. Soderberg (Caltech) and E. Berger (Carnegie Observatories) report: "Digital image subtraction performed on the individual 240-s r-band frames obtained with Gemini/GMOS (GCN 4369) reveal a fading source coincident with the position of the bright NIR (GCN 4367) and optical (GCN 4369) object located within the XRT error circle (GCN 4366). The fading source is offset from the center of the bright object, which we identify as the host galaxy, by about 0.2" to the west. Given the offset and fading behavior we conclude that this object is the afterglow of GRB 051221. Detailed photometry is in progress."