TITLE: GCN CIRCULAR NUMBER: 17933 SUBJECT: V404 Cyg: Swift/UVOT Detection DATE: 15/06/16 15:01:12 GMT FROM: Samantha Oates at MSSL S. R. Oates (IAA-CSIC/UCL-MSSL) reports on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of V404 Cyg 184 s after the BAT trigger (Barthelmy et al., GCN Circ. 17929). A source consistent with the XRT position is detected in the initial UVOT exposures. The white filter is approximately constant for the first 600s after the trigger, but a slight increase in flux is observed in white after this time until observations cease at ~900s. The increase in flux is also observed in the v and b filters. Preliminary detections 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) 184 334 147 15.72 +/- 0.02 white 875 919 44 15.34 +/- 0.03 v 673 693 19 14.58 +/- 0.07 v 848 867 19 14.22 +/- 0.06 b 599 619 19 16.17 +/- 0.08 b 773 793 19 15.78 +/- 0.07 u (FC) 342 591 246 16.13 +/- 0.04 u 748 768 20 16.06 +/- 0.11 w1 722 742 19 > 17.66 m2 698 717 19 > 17.45 w2 649 843 39 > 18.37 The magnitudes in the table are not corrected for the large Galactic extinction due to the reddening of E(B-V) = 2.02 in the direction of the burst (Schlegel et al. 1998).