TITLE: GCN CIRCULAR NUMBER: 6561 SUBJECT: GRB 070621: ROTSE-III Prompt Optical Limits DATE: 07/06/21 23:46:58 GMT FROM: Eli Rykoff at U of Michigan/ROTSE E.S. Rykoff (U Mich), W. Rujopakarn (Steward), B. E. Schaefer (Louisiana State), H. Swan (U Mich), J. Aretakis (U Mich), 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 070621 (Swift trigger 282808, Sbarufatti, et al., GCN 6560), producing images beginning 6.1 s after the GCN notice time. An automated response took the first image at 23:18:03.1 UT, 23.3 s after the burst, and during gamma-ray emission. 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 ongoing. 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 16.4-17.8; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 23:18:03.1 23:18:08.1 5 16.6 23.3 N 23:18:03.1 23:19:20.7 77 17.5 23.3 Y 23:19:28.5 23:24:16.3 287 18.5 108.7 Y 23:24:25.7 23:35:51.6 685 18.8 405.9 Y