TITLE: GCN CIRCULAR NUMBER: 10254 SUBJECT: GRB 091208: ROTSE-III Optical Limits DATE: 09/12/08 09:16:45 GMT FROM: Fang Yuan at ROTSE F. Yuan (U Mich), W. Rujopakarn (Steward), W. Zheng (U Mich), report on behalf of the ROTSE collaboration: ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB 091208 (Swift trigger 378554; E. A. Hoversten et al, GCN 10253), producing images beginning 9.6 s after the GCN notice time. An automated response took the first image at 08:46:30.5 UT, 30.0 s after the burst. We took 10 5-sec and 42 20-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, for both single images and coadding into sets of 10. Individual images have limiting magnitudes ranging from 14.6-16.6; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 08:52:40.1 08:57:21.5 281 17.3 399.6 Y 08:57:30.2 09:02:12.0 281 17.4 689.7 Y