TITLE: GCN CIRCULAR NUMBER: 9967 SUBJECT: GRB 090929B: RAPTOR Discovery of Optical Counterpart DATE: 09/09/29 16:46:34 GMT FROM: James Wren at LANL J. Wren, W.T. Vestrand, P.R. Wozniak, H. Davis, B. Norman of Los Alamos National Laboratory report: The RAPTOR telescope system responded to Swift trigger 371050 (Pagani et al., GCN 9963) under good observing conditions. Our narrow-field instruments began observing the location at 10:20:53.37 UTC, 11.77 minutes after the initial BAT trigger. Our initial images show an R~16.8 optical counterpart at the enhanced XRT location (Page et al., GCN 9965). The object fades steadily reaching R~18.0 at 10:50 UTC. Our unfiltered images were calibrated against the USNO-B1 R-band. Unfortunately, an 11th magnitude star is only 19 arcseconds away, limiting our photometric accuracy. Further analysis is ongoing.