TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3425 SUBJECT: XRF 050509c optical afterglow DATE: 05/05/16 10:55:28 GMT FROM: Jens Hjorth at U.Copenhagen J. Gorosabel (IAA-CSIC), B. L. Jensen (NBI), G. Galaz (Pont. Uni. de Chile), R. Salinas (Uni. de Concepcion), J. Hjorth, J. P. U. Fynbo, H. Pedersen, D. Watson, P. Jakobsson, J. M. Castro Cerón (NBI), report: "We have carried out R-band observations of XRF 050509c (HETE trigger #3751; GCN 3402) error box with the 1.54m Danish telescope (+DFOSC) on La Silla, as follows: ====================================== Date UT t-to Texp Seeing May 2005 (hr) (s) (") -------------------------------------- 10.1545-10.1703 5.1 2x600 1.2 14.0490-14.0829 98.8 4x600 1.3 ====================================== Both observations are centred in the SXC error box covering ~80% of it. Comparison of the two epoch images reveals, well centred in the SXC error box, a fading point-like object with coordinates: RA(J2000) = 12:52:53.94 Dec(J2000) = -44:50:04.1 (internal astrometric error of 1") which we propose as the XRF 050509c optical afterglow. Assuming R = 17.9 for the USNO star placed at (12:52:52.34, -44:49:23.1) we estimate a decay from R~20.1 to R~24.0 between the two epochs. This would imply a power-law decay index of -1.2 typical of GRB afterglows. Finding charts can be found at: http://www.dsri.dk/~jgu/grb050509c/FCs/grb050509c.1.54dk.epoch1.gif http://www.dsri.dk/~jgu/grb050509c/FCs/grb050509c.1.54dk.epoch2.gif Further observations are planned."