TITLE: GCN CIRCULAR NUMBER: 22810 SUBJECT: GRB 180618A: Swift/UVOT Detection DATE: 18/06/20 18:15:17 GMT FROM: Mike Siegel at PSU/Swift MOC M. H. Siegel (PSU) and S. J. LaPorte (PSU) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 180618A 86 s after the BAT trigger (LaPorte et al., GCN Circ. 22790). We confirm the previously reported fading optical afterglow, also detected by LT (Guidorzi et al., GCN Circ. 22792) and MASTER (Tyurina et al., GCN Circ. 22797). We find that the source had faded away completely by the second orbit. The detection of the afterglow in six of UVOT's filters places an upper limit on the redshift of z < 1.2. The preliminary UVOT position is: RA (J2000) = 11:19:45.84 = 169.94102 (deg.) Dec (J2000) = +73:50:13.5 = 73.83707 (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) 86 236 147 17.46 +/- 0.04 white 86 1544 390 17.95 +/- 0.03 white 5680 7315 393 >21.09 v 628 1596 117 18.71 +/- 0.22 v 6090 6290 196 >19.25 b 553 1175 58 18.84 +/- 0.16 b 5475 7110 393 >20.39 u (fc) 298 548 246 17.50 +/- 0.06 u 298 1497 313 17.62 +/- 0.06 u 5270 6905 393 >19.94 w1 676 1472 97 18.59 +/- 0.24 w1 5065 6700 393 >19.63 m2 1254 1621 58 >17.6 w2 604 1398 97 18.85 +/- 0.28 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.07 in the direction of the burst (Schlegel et al. 1998).