TITLE: GCN CIRCULAR NUMBER: 21399 SUBJECT: GRB 170728B: Swift/UVOT Further Analysis DATE: 17/07/31 19:31:50 GMT FROM: Sam LaPorte at PSU GRB 170728B: Swift/UVOT Detection S. J. LaPorte (PSU), S.W.K. Emery (MSSL), and S. B. Cenko (GSFC) report on behalf of the Swift/UVOT team: After detailed analysis of initial data reported in LaPorte and Cenko, GCN Circ. 21388, we do find a significant detection in initial optical filter exposures. The Swift/UVOT began settled observations of the field of GRB 170728B 473 s after the BAT trigger (Cenko et al., GCN Circ. 21371). A fading source consistent with the optical position (D'Avanzo et al. GCN Circ. 21373) is detected in the initial UVOT exposures. 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 876 1026 147 20.56 +/- 0.20 v 630 8278 627 20.23 +/- 0.27 b 728 2561 194 20.60 +/- 0.29 u 703 13868 1276 >21.2 w1 679 19251 2000 >21.6 m2 6847 18681 1279 >22.1 w2 6437 8073 393 >21.5 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.03 in the direction of the burst (Schlegel et al. 1998).