TITLE: GCN CIRCULAR NUMBER: 10323 SUBJECT: GRB 100111A: Swift UVOT Refined Analysis DATE: 10/01/11 21:17:29 GMT FROM: Erik Hoversten at Swift/Penn State E. A. Hoversten (PSU) and L. Vetere (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 100111A 67 s after the BAT trigger (Vetere, et al., GCN Circ. 10317). A source is detected at 17.62 magnitudes in the initial white finding chart at the position RA, Dec (J2000) = 247.04833, 15.55064 which is equivalent to: RA(J2000) = 16:28:11.60 = 247.04833 DEC(J2000) = +15:33:02.3 = 15.55064 with a 90%-confidence error radius of about 0.51 arc sec. This position is 0.8 arcsec from the UVOT-enhanced XRT position (Beardmore, et al. GCN Circ. 10321) and the two positions agree within the error bars. The source faded by more than a magnitude in the white filter over the first orbit of observations. Additionally the source is detected in the b, u, and uvw1 filters, with a marginal 2.4-sigma detection in v and a 2.1-sigma detection in uvm2. Given the detection in the uvw1 filter the redshift of this burst is less than 1.9. The observed magnitudes and 3-sigma upper limits using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627) for the initial exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag ============================================================= white 67 217 147 17.62 +/- 0.05 white 858 1007 147 18.73 +/- 0.10 v 609 1057 58 > 18.41 b 535 727 39 18.76 +/- 0.27 u 279 529 245 17.37 +/- 0.07 uvw1 658 851 38 18.57 +/- 0.36 uvm2 633 827 38 > 18.99 uvw2 584 1033 58 > 19.04 The values quoted above are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.05 in the direction of the burst (Schlegel et al. 1998).