TITLE: GCN CIRCULAR NUMBER: 10036 SUBJECT: GRB 091018: ROTSE-III Detection of Optical Counterpart DATE: 09/10/18 21:34:32 GMT FROM: Brad Schaefer at LSU B. E. Schaefer (Louisiana State) 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 091018 (Swift trigger 373172, Stamatikos et al., GCN 10034). The first image was at 20:57:36.1 UT, 556.6 s after the burst (11.1 s after the GCN notice time). The unfiltered images are calibrated relative to USNO A2.0. We detect a new object, not visible in the DSS (second epoch), with coordinates: 02:08:44.6 -57:32:53.6 (J2000), with positional uncertainty of 1" or better start UT mag mlim(of image) ---------------------------------- 20:57:36.1 15.4 16.4 With this, we confirm the optical afterglow reported in GCN 10034 as a UVOT detection. The source is rapidly fading. A jpeg image is available at http://www.rotse.net/images/gsb373172_3c01_img.jpg Note that the object numbered 14 (in the lower of the two circles) is the candidate in question. Continuing observations are in progress.