GCN Circular 15673

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.