TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3736 SUBJECT: GRB050801: Optical photometry and decay index DATE: 05/08/02 15:01:05 GMT FROM: Brian Lindgren Jensen at U.of Copenhagen GRB050801.769: Optical photometry and decay index J. P. U. Fynbo, B. L. Jensen, J. Hjorth, K. G. Woller, D. Watson (Niels Bohr Institute), P. Fouque (Obs-MiP, Toulouse), M. I. Andersen (Potsdam, AIP) report on behalf of the DARK Cosmology Centre: "We obtained BVR-band imaging of the XRT error circle of GRB050801.769 (Band et al. GCN#3725, Perri et al. GCN#3731) with the Danish 1.5m (La Silla) on Aug. 2.0 UT (approx. 6hr after the burst). We detect the optical afterglow candidate source reported by Rykoff et al. (GCN#3726), with position (RA, Dec)(J2000.0) 13:36:35.363, -21:55:42.03 Based on a preliminary std. star calibration, we obtain the following photometry of the afterglow candidate: Date (UT): Filter: Exptime: Mag. Aug 1.997 B 600s 22.36+-0.05 Aug 2.012 V 600s 21.03+-0.05 Aug 1.989 R 600s 20.75+-0.04 Aug 2.005 R 600s 20.87+-0.04 Aug 2.020 R 600s 20.94+-0.05 (in this calibration, we obtain B=20.95, V=19.74, R=19.63 for the star at: (RA, Dec) = (13:36:33.657, -21:55:29.55)). Fitting a power-law to our three R-band data points gives a lightcurve, which intersects the R=17.29 ROTSE-III (Rykoff et al. GCN#3726) data point at t-t0=1590s, to within the errors of that data point. Including the ROTSE-III data point in the fit, we find an R-band decay slope of -1.29 +-0.05. A findingchart of the field is shown at: http://www.astro.ku.dk/~brian_j/grb/grb050801.769/ "