TITLE: GCN CIRCULAR NUMBER: 12908 SUBJECT: GRB 111228A: Possible host detected by Swift/UVOT DATE: 12/02/02 21:57:17 GMT FROM: Paul Kuin at MSSL Paul Kuin (MSSL/UCL) and Tilan Ukwatta (MSU) reports on behalf of the Swift UVOT Team: Until recently, Swift UVOT continued observations in multiple filters of GRB 111228A (Ukwatta et al., GCN Circ. 12737). Based on the observations taken up to January 31, 2012 in the white filter, a possible host is detected. The UVOT position in decimal degrees of the GRB during the early and peak emission period is: RA=150.06684 DEC=+18.297834, J2000 (sexagesimal 10:00:16.04, +18:17:52.20), consistent with the enhanced XRT position reported in GCN Circ. 12747 by Goad et al.. The position of the late time emission/possible host is: RA=150.06643 DEC=+18.297903, J2000 (sexagesimal 10:00:15.94, +18:17:52.45). The position of the late-time emission is offset by 0.9" which is significantly larger than the position error for a single uvot image of 0.5" (Breeveld et al., 2010, MNRAS 406, 1587), and the unknown smaller position error when comparing between summed UVOT images. The offset suggests that the late-time emission is due to a different source, possibly the host. Other evidence comes from a late time flattening of the light curve. The late time count-rate light curve decays with a power index of -1.3 in nearly all bands. Taking that as the decay rate, the light curves in white and uvw2 deviate at times more than 800ks after the trigger. Extrapolating the power law decay and subtracting from the late-time white count rate, we estimate a host magnitude in white = 24.6 (+0.4/-0.6) mag, which corresponds to a host flux in the broadband white filter of (2.05+/-0.8)x10^-7 Jy (3471 A).