TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5377 SUBJECT: GRB060729: ROTSE-III Observations of the Slow Optical Decay DATE: 06/08/01 03:57:17 GMT FROM: Robert Quimby at U of Texas/ROTSE R. Quimby (U Texas) and E. S. Rykoff (U Mich), report on behalf of the ROTSE collaboration: ROTSE-IIIa, located at the Siding Spring Observatory, Australia, responded to Swift trigger 221755 (GRB 060729; Grupe et al., GCN 5365) with an automatic sequence of 10 5-second, 10 20-second, and 34 60-second integrations before twilight set in (Quimby et al., GCN 5366). Another 30 60-second images were taken on July 30, and 60 further 60-second frames were recorded on July 31. No optical source was detected in our first image, but seconds later we clearly detect a 15.7 magnitude transient object at the location of the UVOT source (GCN 5365; Immler, GCN 5367). The object faded over the next 100 seconds, and then brightened to about 16.5 magnitude around 400 seconds after the BAT trigger. After this, the optical transient slowly began to fade, approximately following a t^-0.23 power-law at least until the July 30 observations. On July 31, the optical transient was about 19.4 magnitude, significantly fainter than an extrapolation of the simple power-law would predict. The 2.9 magnitude drop from t~400-seconds marks one of the slowest 2 day average decline rates ever recorded by ROTSE-III. We set the following representative detections and limits: tstart tend exp mag emag ----------------------------------------------- 64.26 69.26 5 >16.60 92.86 97.86 5 15.67 0.10 187.06 219.96 15 >17.28 448.57 604.68 120 16.56 0.05 2269.84 2830.05 480 16.93 0.05 72997.24 75133.01 1800 18.00 0.18 171304.69 175581.44 3600 19.39 0.18 (times are seconds after the BAT trigger; magnitudes are unfiltered and calibrated against the USNO-B1.0 R2)