TITLE: GCN CIRCULAR NUMBER: 11148 SUBJECT: GRB 100823A: Swift/UVOT Evidence for Fading Afterglow Emission DATE: 10/08/26 19:03:19 GMT FROM: Margaret Chester at PSU M. M. Chester (PSU), T.S. Koch (PSU), E. A. Hoversten (PSU), and V. Mangano (INAF-IASFPA) report on behalf of the Swift/UVOT team: Further examination of Swift/UVOT observations of GRB 100823A show that a source detected in the white finding chart exposure, initially thought to correspond to the DSS2 source mentioned in Levan et al., GCN Circ. 11138, faded in subsequent exposures to an apparently steady level. The source was marginally detected in the u finding chart exposure. The position of the source in both the finding chart and later images is consistent with the enhanced XRT position (Goad et al., GCN Circ. 11136). We propose this result as further evidence that Source C in Levan et al., GCN Circ. 11145, and referenced in Suzuki et al., GCN Circ. 11146, is the afterglow of GRB 100823A. Magnitudes for the white and u finding charts and white optimally co- added exposures are given in the following table. All photometry was performed using the UVOT photometric system (Poole et al. 2008, MNRAS, 383, 627). The values quoted are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.04 in the direction of the burst (Schlegel et al. 1998). Filter T_start(s) T_stop(s) Exp(s) Mag Error S/N white_FC 78 227 147 20.7 +/-0.3 2.9 u_FC 290 539 246 20.4 +/-0.4 2.1 white 576 57266 3157 23.4 +/-0.8 2.3 white 61907 63386 1453 22.9 +/-0.7 2.4 white 67668 73670 2674 23.3 +/-0.8 2.1 white 73673 75876 2168 23.0 +/-0.7 2.5 The UVOT position determined from co-adding all of the later images (after T=56215s) is: RA (J2000) 01:22:49.03 = 20.70429 (deg) Dec (J2000) +05:50:06.4 = +5.83511 (deg) with an estimated uncertainty of 0.9 arcsec (radius, 90% confidence, statistical + systematic).