TITLE: GCN CIRCULAR NUMBER: 16145 SUBJECT: GRB 140423A: ROTSE-III Detection of Optical Counterpart DATE: 14/04/23 10:01:48 GMT FROM: Farley V. Ferrante at Southern Methodist U/ROTSE F. V. Ferrante (SMU), T. Guver (Istanbul Univ.), H. Flewelling (IfA/Hawaii), R. Kehoe (SMU), and G. Dhungana (SMU) report on behalf of the ROTSE GRB team: ROTSE-IIIb, located at McDonald Observatory, Texas, responded to GRB 140423A (Swift trigger 596901, GCN 16142; Elenin et al., GCN 16143; A. Kaur et al., GCN 16144). The first image was at 08:32:45.1 UT, 51.5 s after the burst (0.1 s after the GCN notice time). The unfiltered images are calibrated relative to USNO A2.0. We detect a 14.2 magnitude, fading source with coordinates: 13:09:08.08 +49:50:26.5 (J2000), with positional uncertainty of 1" or better start UT mag mlim(of image) ---------------------------------- 08:33:39.6 14.2 16.0 This source is not visible in DSS (second epoch), 2MASS or the MPChecker database. The object decayed from ~14.2 magnitude to ~15.0 magnitude in ~100 seconds. A jpeg image is available at http://www.rotse.net/images/gsq596901_3b01_img.jpg Continuing observations are in progress.