TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4782 SUBJECT: GRB 060218A: ROTSE-III Possible Optical Flare DATE: 06/02/19 02:51:12 GMT FROM: Robert Quimby at U of Texas/ROTSE R. Quimby (U Texas), B. E. Schaefer (Louisiana State), H. Swan (U Mich), report on behalf of the ROTSE collaboration: ROTSE-IIIb, located at the McDonald Observatory, Texas, responded to Swift trigger 191157 (possible GRB 060218A; Cusumano et al. GCN 4775; Barbier et al. GCN 4780). The first image was taken at 03:36:07.9 UT, 97.0 s after the burst (5.9 s after the GCN notice time). We detect a source coincident with the refined UVOT position reported by Marshall et al. (GCN 4779). Intermittent clouds reduced the overall data quality, and after 80 minutes the clouds rendered the images unusable. We set the following magnitude limits and detection calibrated relative to the USNO-B1.0 R2 magnitudes: tstart(s) tend(s) exp(s) mag emag limmag -------------------------------------------------------------- 96.98 130.61 25 --- -- 15.68 217.71 472.78 180 --- -- 17.60 482.43 681.24 180 --- -- 18.55 690.49 1027.40 300 18.09 0.11 19.00 1036.55 1789.28 660 --- -- 18.44 1868.39 4780.74 1680 --- -- 16.91 These magnitudes show that the optical source was significantly brighter for a ~5-minute interval starting ~12-minutes after the burst than either before or after this interval, as is consistent with the two UVOT V-band magnitudes reported by Marshall et al. (GCN 4779).