TITLE: GCN CIRCULAR NUMBER: 11765 SUBJECT: Swift/UVOT observations of GRB110223b DATE: 11/02/24 13:46:54 GMT FROM: Massimiliano de Pasquale at MSSL-UCL Swift UVOT observations of GRB110223b M. De Pasquale (MSSL-UCL) and M. Stamatikos (GSFC) report, on behalf of the Swift team: Swift/UVOT began settled exposures of GRB110223b (Stamatikos et al. GCN circ. 11754) 77 s after the trigger with a white finding chart. There is an uncatalogued source 1.5 arcseconds from the enhanced XRT afterglow position (Beardmore et al., GCN circ. 11758). This position is also consistent with the optical transient found by GROND (Sudilovsky et al., GCN circ. 11756) This source, which we identify as the optical afterglow of the burst, is detected in white, v, b, u. Magnitudes and 3 sigma upper limits, derived using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627), are shown in the table below. We caution that the photometry is complicated by a crowded field. Filter T_start(s) T_end(s) exp Magnitude white (FC) 77 227 147.4 18.98 ± 0.10 white 620 814 39.0 19.70 ± 0.30 white 870 1546 206.0 19.61 ± 0.13 v 620 1596 117.3 17.88 ± 0.15 b 546 1522 97.4 19.57 ± 0.30 u (FC) 290 540 245.8 19.20 ± 0.19 u 694 1496 77.8 >19.30 uw1 670 1471 97.4 >19.16 um2 645 1445 97.4 >18.78 uw2 596 1572 116.7 >19.16 These magnitudes are not corrected for the strong Galactic extinction along the line of sight, corresponding to E(B-V) = 0.23 (Schlegel et al. 1998)