TITLE: GCN CIRCULAR NUMBER: 22611 SUBJECT: GRB 180404B: Swift/UVOT Detection DATE: 18/04/06 18:14:06 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 180404B 82 s after the BAT trigger (LaPorte et al., GCN Circ. 22590). A fading source consistent with the XRT position (Beardmore et al. GCN Circ. 22594) is detected in the initial UVOT exposures. The preliminary UVOT position is: RA (J2000) = 03:33:34.23 = 53.39262 (deg.) Dec (J2000) = -50:12:54.2 = -50.21506 (deg.) with an estimated uncertainty of 0.46 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) 82 231 147 18.66 +/- 0.09 white 82 1720 411 19.39 +/- 0.09 white 6421 6620 196 >20.7 v 624 7031 510 >19.5 b 550 7661 322 >20.3 u (fc) 295 545 245 19.49 +/- 0.31 u 295 1670 343 19.70 +/- 0.29 u 6011 7648 393 >20.1 w1 674 7442 510 >20.6 m2 1600 7236 413 >20.5 w2 600 6826 452 >20.2 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.01 in the direction of the burst (Schlegel et al. 1998).