TITLE: GCN CIRCULAR NUMBER: 10871 SUBJECT: GRB 100621A: ROTSE-III Optical Limits DATE: 10/06/21 03:40:05 GMT FROM: Shashi Bhushan Pandey at ROTSE S. B. Pandey (U Mich), W. Rujopakarn (Steward), T. Guver (U Arizona), W. Zheng (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 100621A (Swift trigger 425151; Ukwatta T. N., GCN 10870), producing images beginning 5.0 s after the GCN notice time. An automated response took the first image at 03:04:05.0 UT, 32.2 s after the burst, and during the gamma-ray emission, under fair 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; the field is not crowded. Individual images have limiting magnitudes ranging from 16.1-17.3; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 03:04:05.0 03:04:10.0 5 16.1 32.2 N 03:05:30.0 03:09:13.0 200 18.4 232.2 Y