TITLE: GCN CIRCULAR NUMBER: 10284 SUBJECT: GRB 091221: ROTSE-III Detection of Optical Counterpart DATE: 09/12/21 21:37:02 GMT FROM: Shashi Bhushan Pandey at ROTSE GRB 091221: ROTSE-III Detection of Optical Counterpart W. Zheng, F. Yuan and 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 091221 (Swift trigger 380311, GCN 10283 Krimm et al.). The first image was at 20:53:18.1 UT, 25.8 s after the burst (9.1 s after the GCN notice time). The unfiltered images are calibrated relative to USNO A2.0. We detect a 17.4 magnitude, source with coordinates: 03:43:11.47 +23:14:29.38 (J2000), with positional uncertainty of 1" or better start UT mag mlim(of image) ---------------------------------- 20:54:49.5 17.4 18.0 This source is not visible in DSS (second epoch), 2MASS or the MPChecker database. A jpeg image is available at http://www.rotse.net/images/gsb380311_3c011-020_key.jpg, the candidate is marked as 110. Continuing observations are in progress.