GCN Circular 3878
The Swift Ultra-Violet/Optical Telescope (UVOT) observations began at 00:53:49 UT, 1.8 hrs after the BAT trigger. UVOT detects a fading source at a position consistent with that reported by Gorosabel et al. (GCN 3865) in 5 of its 6 filters. The magnitudes or 3-sigma upper limit in the first exposure taken in each filter are as follows: Filter Mid Time (hrs) Exp (s) Mag/3-sig UL V 1.8 109 20.02+/-0.39 B 5.1 305 20.88+/-0.36 U 3.6 217 19.55+/-0.32 W1 9.5 205 19.85+/-0.33 M2 11.7 121 19.34+/-0.36 W2 12.3 1457 > 20.79 The afterglow has faded below 2-sigma above background 11hrs after the burst trigger in all filters apart from the V and U, where it is still detected at the 2.3-sigma and >3-sigma level, respectively. The magnitudes have not been corrected for extinction. These magnitudes are based on preliminary zero-points, measured in orbit, and will require refinement with further calibration.