TITLE: GCN CIRCULAR NUMBER: 10289 SUBJECT: GRB 091221: Swift/UVOT refined analysis DATE: 09/12/22 13:58:52 GMT FROM: Massimiliano de Pasquale at MSSL-UCL M. De Pasquale (MSSL/UCL) and H. Krimm (CRESST/GSFC/USRA) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB091221 80s after the BAT trigger with a 150s finding chart exposure in the White filter. We detect the optical afterglow in the finding chart exposure. The refined UVOT position is RA = 55.79750, Dec = 23.24119 (J2000), corresponding to RA = 03h 43m 11.40s, Dec = +23d 14m 28.3s (J2000) with an estimated uncertainty of 0.6 arcsec (radius, 90% confidence). This position is consistent with the XRT enhanced position (Evans et al., GCN 10287) and ground-based detections (Zheng et al., GCN 10284; Klose et al., GCN 10286). The afterglow is not detected anymore in single or summed exposures in all filters. Preliminary magnitudes and 3 sigma upper limits are reported below. Filter T_start(s) T_stop(s) Exposure(s) Mag Err ----------------------------------------------------------------------- white 80 230 149.8 20.19 +/- 0.27 white 3657 11605 927.1 >21.62 v 4068 5704 393.2 >19.56 b 3452 11054 1278.4 >21.11 u 4683 10142 1083.4 >20.68 uvw1 4478 6075 354.4 >19.96 uvm2 4273 5909 393.3 >19.79 uvw2 3863 5499 393.3 >20.07 ------------------------------------------------------------------------ The values quoted above are not corrected for the heavy Galactic extinction corresponding to the reddening of E(B-V) = 0.22 in the direction of the burst (Schlegel et al. 1998, ApJS, 500, 525). The photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383, 627). [GCN OPS NOTE(22dec09): Per author's request, the Subject-line was changed from 091208B to 091221.]