TITLE: GCN CIRCULAR NUMBER: 15673 SUBJECT: GRB 131231A: Swift/UVOT Observations of the Optical Afterglow DATE: 14/01/03 19:47:04 GMT FROM: Stephen Holland at STScI S. T. Holland (STScI) and V. Mangano (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began observations of the field of GRB 131231A 136.5 ks after the Fermi/LAT trigger (Sonbas et al., 2013, GCNC 15640). The preliminary UVOT position is RA (J2000) 00:42:21.66 = 10.59025 (deg) Dec (J2000) -01:39:10.6 = -1.65294 (deg) with an estimated uncertainty of 0.43 arcsec (radius, 90% confidence, statistical + systematic). We detect the optical afterglow at the position reported by Xu et al. (2013, GCNC 15641). Preliminary UVOT photometry is presented below. ------------------------------------------------------ Filter TSTART TSTOP Exposure Mag Err ------------------------------------------------------ u 246,875 248,554 1652 19.57 0.10 white 136,762 138,942 1650 19.37 0.05 white 228,985 231,172 1639 20.07 0.07 ------------------------------------------------------ The quoted magnitudes and upper limits have not been corrected for the Galactic extinction along the line of sight to this burst of E_{B-V} = 0.02 mag (Schlafly et al. 2011, ApJS, 737, 103). The photometry is in the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373). The white observations yield a power-law decay index of alpha ~ 1.25 between 1.6 and 2.7 days. This is consistent with that found by Perley et al. (2014, GCNC 15650) between 1.0 and 1.9 days. The power-law decay index in the X-ray band at this time is alpha ~ 1.6 (Mangano et al. 2014, 15648), suggesting that there may be a cooling break between the optical and X-ray regimes.