TITLE: GCN GRB OBSERVATION REPORT NUMBER: 1275 SUBJECT: GRB020305: Properties of the Candidate Optical Afterglow DATE: 02/03/19 02:12:18 GMT FROM: Don Lamb at U.Chicago B. C. Lee, D. Q. Lamb, D. L. Tucker, D. E. Vanden Berk, J. Krzesinski, D. Long, P. R. Newman, A. Nitta, and S. A. Snedden, on behalf of the SDSS GRB team, report: We observed the field of GRB020305 (= H1939) (Ricker et al., GCN 1262) using the SDSS 0.5-m "Photometric Telescope" (PT) at APO under partly cloudy conditions beginning at UTC 2002 March 6.33 (20 hours after the GRB). We obtained three sets of three 400-second g'-band, r'-band, and i'-band exposures (41.5' x 41.5' field of view) centered on the reported best-fit location of GRB020305 (Ricker et al., GCN 1262). These exposures covered the entire combined HETE/IPN error box for the burst (Hurley et al., GCN 1263). Co-adding the images taken in each filter, we find an object with estimated magnitudes of g* = 20.3 +/- 0.5, r* = 20.1 +/- 0.5, and i* = 19.8 +/- 0.5, within 1 arcsec the location of the candidate optical afterglow reported by Price et al. (GCN 1267). These errors are conservative, and reflect the fact that we do not yet fully understand our systematics. The magnitudes we measure are consistent with a power-law spectrum having a slope of roughly -1, which is typical of GRB afterglows. Combining our results with the limiting magnitude of R ~ 21.5 at UTC 2002 Mar 9.34 reported by Price et al. (GCN 1267) and the magnitudes of R = 22.6 and I = 22.1 at UTC 2002 Mar 12.6 reported by Ohyama et al. (GCN 1271), we find that the candidate afterglow faded between UTC 2002 Mar 6.33 and Mar 12.6 at a rate that is consistent with a temporal slope of roughly -1.3, which is also typical of GRB afterglows. Thus both the spectral and temporal behavior of the candidate afterglow identified by Price et al. (GCN 1267) support its identification as the optical afterglow of GRB020305. This message may be cited.