TITLE: GCN CIRCULAR NUMBER: 10655 SUBJECT: Swift Trigger 420256: ROTSE-III Optical Limits DATE: 10/04/23 03:31:22 GMT FROM: Limin Xiao at Louisiana State U L. Xiao (Louisiana State), W. Zheng (U Mich), H. Flewelling (IfA/Hawaii), report on behalf of the ROTSE collaboration: ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, responded to Swift trigger 420256, producing images beginning 7.5 s after the GCN notice time. An automated response took the first image at 02:45:32.5 UT, 90.2 s after the burst. 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, for both single images and coadding into sets of 10. Individual images have limiting magnitudes ranging from 15.9-17.3; we set the following specific limits. start UT end UT t_exp(s) mlim t_start-tGRB(s) Coadd? -------------------------------------------------------------------- 02:45:32.5 02:46:48.9 76 17.1 90.2 Y 02:47:03.3 02:51:45.9 282 17.7 181.0 Y 02:51:54.6 03:03:15.0 680 17.9 472.3 Y