TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5222 SUBJECT: GRB 060605: ROTSE-III Optical Counterpart DATE: 06/06/05 20:04:25 GMT FROM: Brad Schaefer at LSU B. E. Schaefer (Louisiana State), E.S. Rykoff (U Mich), D.A. Smith (Guilford), R. Quimby (U Texas), report on behalf of the ROTSE collaboration: ROTSE-IIIa, located at Siding Spring Observatory, Australia, found a bright optical transient (Rykoff et al. 2006, GCN 5220) near the center of the Swift BAT position for GRB 060605A (M. J. Page et al. 2006, GCN 5221). The Swift XRT and UVOT later reported a fading counterpart at this same position (Page et al. 2006, GCN 5221). In the first ROTSE image (started 48.7 s after the burst) the source was fainter than 16.5 mag (calibrated against USNO stars in the R-band). The source rapidly brightened to roughly 15.7 mag by 150 seconds, and after a short plateau has since been fading fast. Our light curve shows some apparently significant fluctuations starting ~1400 seconds after the burst. The decline after 1000 seconds roughly follows a power law with index of -0.8. A jpeg image is available at http://www.rotse.net/images/gsb213630_3a011-020_key.jpg The optical transient is indicated by the central yellow circle labelled as object 44, while the blue circles represent stars in the USNO B1.0 catalog. Continuing observations are in progress.