TITLE: GCN CIRCULAR NUMBER: 9462 SUBJECT: GRB 090531B: ROTSE-III Optical Limits DATE: 09/05/31 19:20:55 GMT FROM: Brad Schaefer at LSU B. E. Schaefer (Louisiana State), T. Guver (U Arizona), 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 090531B (Swift trigger 353728; J. R. Cummings et al., GCN 9461), producing images beginning 7.3 s after the GCN notice time. An automated response took the first image at 18:36:22.3 UT, 26.1 s after the burst, and during the gamma-ray emission, under excellent conditions. We took 10 5-sec, 10 20-sec and 50 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, for both single images and coadding into sets of 10; however, we are limited as the field is somewhat crowded. Individual images have limiting magnitudes ranging from 15.3-16.0; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 18:36:22.3 18:36:27.3 5 15.3 26.1 N 18:47:31.3 18:52:11.6 280 16.3 695.1 Y [GCN OPS NOTE(31may09): Per author's request, the "B" was added to the burst name.]