TITLE: GCN CIRCULAR NUMBER: 11198 SUBJECT: GRB 100901A: Lightbuckets Optical Observations DATE: 10/09/03 20:02:34 GMT FROM: Tilan Ukwatta at GSFC/GWU T. N. Ukwatta (GWU/GSFC), T. Sakamoto (GSFC/UMBC), N. Gehrels (GSFC), and K. S. Dhuga (GWU) We observed the Swift GRB 100901A (Immler et al., GCN 11159) with the Lightbuckets 0.61m rental telescope LB-0001 in Rodeo, NM, USA. Under good weather conditions, observations were carried out in the R filter starting Sep. 02 (UT) 07:05 (~ 17.5 hours after the GRB trigger) and a followup observation was performed on Sep. 03 (UT) 07:01 under acceptable weather conditions. The burst afterglow is clearly detected with a R magnitude of ~18.05 in the first observation. The afterglow, not corrected for Galactic extinction, is calibrated against the USNO-B1.0 R1 mag catalog. Time after trigger Exposure (s) R Magnitude 17.5 hours (0.7 days) 3 x 60 ~ 18.05 41 hours (1.7 days) 10 x 60 > 19.1 We acknowledge the helpful assistance of the Lightbuckets staff, in particular LightBuckets CEO Stephen G. Cullen.