TITLE: GCN CIRCULAR NUMBER: 8933 SUBJECT: GRB090305: GMOS-S Candidate Optical Afterglow DATE: 09/03/05 10:32:47 GMT FROM: S. Bradley Cenko at Caltech S. B. Cenko, B. E. Cobb, D. A. Perley, and J. S. Bloom (UC Berkeley) report on behalf of a larger collaboration: We have imaged the field of the short GRB 090305 (Beardmore et al., GCN 8932) with GMOS mounted on the Gemini South telescope beginning approximately 27 minutes after the burst. Observations were taken in the Sloan g', r', and i' filters and cover nearly the entire BAT error circle. Inside the BAT error circle, we find a fading point source at the location (J2000.0): RA: 16:07:07.61 Dec: -31:33:21.9 Calibrating with respect to the USNO-B1 object at 16:07:06.58, -31:33:37.9 (R2=18.99), we measure an initial magnitude of R ~ 23.3. The source fades by approximately 1.1 mag over the next two hours, corresponding to a power-law decay index of ~ 0.6 (assuming no contribution from an underlying host galaxy). At this point we tentatively consider this source the afterglow of GRB090305, although we await confirmation from the XRT. We wish to thank the staff at Gemini South, in particular Peter Pessev, for performing our target of opportunity observations.