TITLE: GCN CIRCULAR NUMBER: 23545 SUBJECT: GRB 181213A: Swift/UVOT Observations DATE: 18/12/18 20:20:49 GMT FROM: Mike Siegel at PSU/Swift MOC M. H. Siegel (PSU) and P. A. Evans (U. Leicester) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 181213A 2352 s after the BAT trigger (Evans et al., GCN Circ. 23525). A fading optical afterglow consistent with initial reports (Lipunov et al., GCN Circ. 23526, Hu et al., GCN Circ. 23529) was detected in four of UVOT’s filters. The lack of detection of this bright burst in the bluer filters would imply a redshift of z > 1.4, which is not inconsistent with the spectroscopic redshift of z < 2.4 from Heintz et al. (GCN Circ. 23537). The preliminary UVOT position is: RA (J2000) = 16:33:04.86 = 248.27025 (deg.) Dec (J2000) = +78:29:48.7 = 78.49685 (deg.) with an estimated uncertainty of 0.43 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 (fc) 2353 2503 294 17.33+/-0.04 white 2633 2653 19 17.15+/-0.10 white 8535 9061 513 19.03+/-0.07 white 66263 67021 738 >21.71 v 2683 2703 19 >17.35 v 26217 37929 1168 >20.45 b 2608 3393 87 17.82+/-0.10 b 8330 15947 230 19.18+/-0.23 b 65351 82801 1115 >20.39 u 2584 3318 216 17.25+/-0.06 u 14999 33141 1327 19.69+/-0.10 u 71983 72421 426 >20.32 uvw1 2559 3113 216 17.50+/-0.11 uvw1 14093 32991 2656 20.07+/-0.16 uvw1 71077 71977 885 >19.97 uvm2 2708 2908 196 >19.26 uvm2 19555 32084 2047 >20.84 uvw2 2659 2679 19 >17.50 uvw2 25310 37634 1771 >20.67 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.04 in the direction of the burst (Schlegel et al. 1998).