TITLE: GCN CIRCULAR NUMBER: 8337 SUBJECT: GRB 081007: Early RAPTOR measurements of optical counterpart DATE: 08/10/07 19:56:45 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 330856 (Baumgartner et al., GCN 8330) under fair observing conditions. Our narrow-field instruments began observing the location at 05:24:17.13 UTC, 24.4 s after the Swift trigger and during the end of the Gamma-Ray emitting interval. We detect the optical counterpart reported by Baumgartner et al. (GCN 8330) and Covino et al. (GCN 8331). We first detect the counterpart at R~16.4 at approximately 90s after the Swift trigger time. The counterpart brightens over the next 50 seconds to magnitude R~15.4 and then begins fading steadily. Our unfiltered images were calibrated against the USNO-B1 R-band. The following table gives selected observations, not corrected for extinction, from this event. t-mid(s) exp(s) mag mag-err -------------------------------------------- 90.31 5.0 16.43 0.22 139.58 10.0 15.38 0.06 367.83 30.0 16.75 0.20 685.42 30.0 17.28 0.19