TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3089 SUBJECT: GRB 050306: optical afterglow DATE: 05/03/12 18:46:02 GMT FROM: Daniele Malesani at SISSA-ISAS,Trieste,Italy P. D'Avanzo, D. Fugazza, S. Covino (INAF-OABr), D. Malesani (SISSA), N. Masetti, E. Palazzi (IASF-CNR), L.A. Antonelli, G.L. Israel (INAF-OARm), and G. Andreuzzi (INAF-TNG), report on behalf of a larger Italian collaboration: We imaged again (Fugazza et al., GCN 3078) the field of GRB 050306 (Markwardt et al., GCN 3071) with the Italian TNG telescope. Observations were carried out under a clear sky and with a seeing of about 1.6", starting on 2005 Mar 9.247 UT. Seven R-band exposures were acquired, with an overall exposure time of 21 minutes (mean UT Mar 9.260, i.e. 3.11 days after the burst). Inside the revised error circle of the XRT afterglow (Perri et al., GCN 3075; Mangano et al., GCN 3086), we detect several sources. After performing PSF photometry with DAOPhot, we find that the object at coordinates: alpha(J2000) = 18:49:13.95 delta(J2000) = -09:09:08.6 (that is, 3.5" off the XRT centroid position) has faded by 0.54+-0.10 mag since our previous observation, carried out ~2.1 days after the GRB (Fugazza et al., GCN 3078). Assuming a powerlaw decay (F(t) = K*t^-delta), this corresponds to delta = 1.28+-0.24, a value typical among GRB afterglows at comparable epochs. We thus propose that this object is the optical afterglow of GRB050603. Based on calibration upon USNO stars, the object has R ~ 23 at the time of our first observation, Mar 8.25 UT. A finding chart of the field is posted at the following URL: http://www.sissa.it/~malesani/GRB/050306 Further observations are planned at the TNG in the following nights, but no spectroscopy will be possible. We warmly acknowledge the TNG staff for carefully performing the observations. This message can be cited.