TITLE: GCN CIRCULAR NUMBER: 10285 SUBJECT: GRB 091221: UVOT Confirmation of an Optical Afterglow DATE: 09/12/21 22:12:23 GMT FROM: Stephen Holland at USRA/NASA/GSFC/SSC S. T. Holland (CRESST/GSFC/USRA), M. de Pasquale (MSSL), P. Kuin (MSSL), and H. A. Krimm (CRESST/GSFC/USRA) report on the behalf of the Swift/UVOT team: The Swift/UVOT observed the field of GRB 091221 (Krimm et al. 2009, GCN Circ. 10283) starting approximately 80 s after the BAT detection. We confirm the optical afterglow detected by Krimm et al. 2009, GCN Circ 10283 and Zheng et al. 2009, GCN Circ. 10284. The preliminary UVOT position is RA = 03:43:11.47 = 55.79771 deg Dec = +23:14:28.9 = +23.24136 deg with a preliminary error estimate of 0.6 arcsec (radius, 90% containment). The preliminary white magnitude is Filter TSTART TSTOP EXPOSURE Mag Err --------------------------------------------------------- White 80 230 150 20.22 0.28 --------------------------------------------------------- The quoted upper limits have not been corrected for the expected Galactic extinction along the line of sight corresponding to a reddening of E_{B-V} = 0.22 mag (Schlegel et al., 1998, ApJS, 500, 525). All photometry is on the UVOT photometry system described in Poole et al. (2008, MNRAS, 383, 627).