TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3720 SUBJECT: GRB 050730: early TAROT optical observations DATE: 05/08/01 08:18:52 GMT FROM: Michel Boer at Obs Haute Prov. Klotz, A. (CESR-OMP), Boer M. (OHP), and Atteia, J.L. (LAT-OMP) report: We imaged the entire field of GRB 050730 detected by SWIFT (Holland et al. GCNC 3704) with the TAROT robotic telescope (D=25cm) located at the Calern observatory, France. Observations started 7.5 seconds after the GCN notice (and 66.3 s after the GRB). The field had an elevation of 29 degrees above horizon at the begining of the observations and then decreased. The first three images taken between 2005-07-30T19:59:29.53 (GRB + 66.3s) and 2005-07-30T20:00:27.38 (GRB + 124.2s) were co-added. The afterglow is perhaps detected very marginally. The magnitude is estimated to be R=15.5 +/- 0.4 comparing with the USNO-B1 nearby stars. This magnitude should be rather considered as an upper limit. A second set of three images taken between 2005-07-30T20:00:33.60 (GRB + 130s) and 2005-07-30T20:01:46.25 (GRB + 203s) we co added. The afterglow is not detected, suggesting R>15.9 comparing with the USNO-B1 nearby stars. Co addition of the six first images (GRB + 66.3s to 203s) shows the afterglow measured to be R=16.24 +/- 0.41. Later images were taken with longer exposure times and were co added to measure the afterglow magnitude with a reasonable accuracy: t=GRB + 26.79 min R=17.00 +/- 0.41 t=GRB + 37.61 min R=17.30 +/- 0.44 t=GRB + 48.61 min R=17.34 +/- 0.37 t=GRB + 61.20 min R=17.92 +/- 0.42 t=GRB + 78.47 min R=17.54 +/- 0.39 t=GRB + 97.40 min R=18.74 +/- 0.53 This later series gives a decay index of 0.89 +/- 0.38. An extrapolation towards the early measurement shows that a break or a re-brightening should occured considering our R=16.24 measurement at t=GRB + 2.31 min. Further informations (light curve and images) on: http://www.cesr.fr/~klotz/grb050730 This message can be cited. -- Michel Boer Observatoire de Haute Provence F - 04870 Saint Michel l'Observatoire 04 92 70 64 59 - Michel.Boer@oamp.fr