GCN Circular 314
J. Kaluzny (Warsaw Univ. Obs.), P. M. Garnavich, K. Z. Stanek (Harvard-Smithsonian CfA), W. Pych (Warsaw Univ. Obs.) and I. Thompson (Carnegie Obs.) report: We measured the object at RA 13:38:07.64, Decl -80:29:48.8 (J2000) (coordinates based on the GSC catalog), reported by Vreeswijk et al. (GCN 310) to be a possible optical counterpart to GRB 990510 in our R-band images taken with the 1.0-meter Swope telescope at the Las Campanas Obs. We find that the object declined by 0.34 mag over 4.6 hours beginning May 10.995 (UT) in the R-band. The data is consistent with a power-law decline with decay index of 0.89, obtained by fitting combined data of Galama et al. (GCN 313) and ours. The comparison star (end figures of 00.82, 11.7) assumed by Galama et al. to be R=17.2 is actually given in the USNO A2.0 catalog as R=16.5. We find the R magnitudes in this region of the USNO catalog to be of dubious quality, especially fainter than 16.5. It is important to obtain an independent photometric calibration for this field. Adding seven 300s exposures in R with approximately 1" seeing reveals an object 1.8" south of the OT and about 3 magnitudes fainter which may be the host galaxy. Compared to nearby field stars, the OT appears significantly bluer by approximately 0.4 mag in V-R color. The R-band lightcurve and the combined image of the field are available at http://cfa-www.harvard.edu/~peterg/grb.html. This message is citeable.