TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3107 SUBJECT: GRB 050306: ROTSE-III early limits DATE: 05/03/16 23:13:55 GMT FROM: Robert Quimby at U of Texas/ROTSE R. M. Quimby, E. S. Rykoff, B. E. Schaefer, D. A. Smith and S. A. Yost report on behalf of the ROTSE collaboration: ROTSE-IIIc, located at the H.E.S.S. site at Mt. Gamsberg, Namibia, responded automatically to Swift GRB 050306 (Markwardt et al., GCN 3071) with the first 5-second exposure beginning Mar 6, 03:34:16.7 UT, 64.8 seconds after the burst. Comparison to the DSS (second epoch) reveals no new sources, but we are limited by crowding in this low galactic latitude field. Using software provided by the Supernova Cosmology Project, we have subtracted ROTSE-IIIc data obtained on Mar. 8 from our first Mar 6 image. No sources are detected on the subtracted frame within the Swift XRT error circle (Perri et al., GCN 3075). In particular, we set a 3-sigma upper limit on the magnitude of 15.8 (unfiltered, relative to USNO A2.0) at the location of the TNG source (D'Avanzo et al., GCN 3089). An extrapolation of a simple power-law with an index of 1.28 from the TNG detection back to the epoch of our first image would predict a ~12 mag source. However, our non-detection is consistent with other optical light curves that are underluminous at early times such as GRBs 030418 and 030723 (Rykoff et al., 2004 ApJ 601,1013)