TITLE: GCN CIRCULAR NUMBER: 6198 SUBJECT: GRB 070311: RAPTOR Detection of Optical Emission DATE: 07/03/12 04:03:22 GMT FROM: James Wren at LANL J. Wren, W.T. Vestrand, P.R. Wozniak, R. White, J. Pergande of Los Alamos National Laboratory report: Our Raptor telescopes responded to Integral trigger 6146 (Mereghetti et al., GCN 6189) at 01:53:16.10 UT, 26.1 s after the trigger and 5.9 s after receiving the GCN packet. We detected optical emission from the IR counterpart reported by Covino et al. (GCN 6190) and confirmed by Bloom et al. (GCN 6191). Due to twilight conditions we did not detect the optical counterpart while the gamma-rays were still being detected by Integral (3-sigma limiting magnitude R~15.5). As the twilight period ended we detected the counterpart at magnitude 17.8 +/- 0.2 in a 40 s image stack with a mid-exposure time of 01:55:22.7 UT. Subsequent image stacks show fading of the counterpart. Our unfiltered magnitudes were calibrated using the R-band magnitudes from the USNO B1.0 catalog.