TITLE: GCN CIRCULAR NUMBER: 18766 SUBJECT: GRB 151228B: Swift/UVOT Detection DATE: 15/12/30 07:45:04 GMT FROM: Alice Breeveld at MSSL-UCL A.A. Breeveld (UCL-MSSL), S.W.K. Emery (UCL-MSSL) and A.Y. Lien (GSFC/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 151228B 49 s after the BAT trigger (Lien et al., GCN Circ. 18739). A fading source consistent with the XRT position (Goad et al. GCN Circ. 18743) is detected in the initial UVOT exposures in the white filter. The preliminary UVOT position is: RA (J2000) = 22:57:41.87 = 344.42446 (deg.) Dec (J2000) = +08:04:53.7 = 8.08159 (deg.) with an estimated uncertainty of 0.58 arc sec. (radius, 90% confidence). Preliminary detections and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 70 220 147 19.62 ± 0.19 v 49 6132 306 >19.2 b 537 5514 274 >20.0 u 282 5309 520 >20.1 w1 661 6373 303 >20.1 m2 6137 6337 197 >19.9 w2 1386 5927 216 >20.8 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.06 in the direction of the burst (Schlegel et al. 1998).