TITLE: GCN CIRCULAR NUMBER: 10149 SUBJECT: GRB 091109B: ROTSE-III Optical Limits DATE: 09/11/09 22:25:08 GMT FROM: Brad Schaefer at LSU B. E. Schaefer (Louisiana State), E.S. Rykoff (UCSB), T. Guver (U Arizona), H. Flewelling (IfA/Hawaii), S. B. Pandey (U Mich), report on behalf of the ROTSE collaboration: ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, responded to GRB 091109B (Swift trigger 375409; S. R. Oates et al., GCN 10148), producing images beginning 9.4 s after the GCN notice time. An automated response took the first image at 21:49:24.5 UT, 21.6 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; the field is not crowded. Individual images have limiting magnitudes ranging from 15.9-17.0; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 21:49:24.5 21:49:29.5 5 16.0 21.6 N 21:49:24.4 21:50:40.5 76 16.9 21.5 Y