TITLE: GCN CIRCULAR NUMBER: 6729 SUBJECT: GRB 070809: ROTSE-III Optical Limits DATE: 07/08/09 19:56:11 GMT FROM: Eli Rykoff at U of Michigan/ROTSE E.S. Rykoff (U Mich), F. Yuan (U Mich), C. Akerlof (U Mich), H. Swan (U Mich), B. E. Schaefer (Louisiana State), R. Quimby (U Texas), report on behalf of the ROTSE collaboration: ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, responded to GRB 070809 (Swift trigger 287344; Marshall, et al., GCN 6728), producing images beginning 10.2 s after the GCN notice time. An automated response took the first image at 19:22:48.2 UT, 30.9 s after the burst, under excellent conditions. We took 10 5-sec, 10 20-sec and 10 60-sec exposures. These unfiltered images are calibrated relative to USNO A2.0 (R). Imaging is on going. Comparison to the DSS (second epoch) reveals no new sources within the 3-sigma Swift/BAT error circle or the XRT error circle, for both single images and coadding into sets of 10. Individual images have limiting magnitudes ranging from 15.8-17.4; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 19:22:48.2 19:22:53.2 5 16.1 30.9 N 19:22:48.2 19:24:06.2 78 17.5 30.9 Y 19:24:19.9 19:29:06.8 286 18.1 122.6 Y 19:29:16.6 19:40:41.9 685 18.4 419.3 Y