TITLE: GCN CIRCULAR NUMBER: 8536 SUBJECT: GRB 081121: ROTSE-III Detection of Optical Counterpart DATE: 08/11/21 20:41:23 GMT FROM: Fang Yuan at ROTSE F. Yuan (U Mich), W. Rujopakarn (Steward) report on behalf of the ROTSE collaboration: ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, responded to GRB 081121 (Swift trigger 335105). The first image was at 20:36:30.0 UT, 57.2 s after the burst (8.9 s after the GCN notice time). The unfiltered images are calibrated relative to USNO A2.0. We detect a 11.6 magnitude, fading source with coordinates: 05:57:06.1 -60:36:10.3 (J2000), with positional uncertainty of 1" or better start UT mag mlim(of image) ---------------------------------- 20:36:29.9 11.6 15.7 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/gsb335105_3c00_img.jpg Note that the object marked 4 is the candidate in question. Continuing observations are in progress.