TITLE: GCN CIRCULAR NUMBER: 23241 SUBJECT: GRB180914B: Swift UVOT detection of an optical afterglow DATE: 18/09/16 16:05:06 GMT FROM: Paul Kuin at MSSL N. Paul M. Kuin reports on behalf of the Swift-UVOT team: Swift-UVOT has performed follow-up observations of the Fermi/LAT-detected burst GRB 180914B (Bissaldi et al. GCN Circ. 23232) in a series of observations tiled on the sky. Within the error of the location of the XRT source (D'Ai, GCN Circ. 23236) we detect a source that appears to be fading. The UVOT source location is RA = 332.2609, Dec = 24.5823 deg which is RA = 22:09:25.53 (J2000) Dec = +25:03:43.8 (J2000) which is consistent with Zheng and Filipenko (GCN Circ. 23237). The preliminary magnitude using the UVOT photometric calibration (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) is filter Tstart Tend exposure magnitude u 8.35e4 8.37e4 76.5 18.90 +- 0.23 u 8.70e4 8.74e4 373.0 18.87 +- 0.11 u 9.27e4 9.29e4 1.93.8 19.05 +- 0.17A Times in seconds since Tzero=2018-09-14 18:22:40 UT. No correction has been made for Galactic interstellar reddening of E(B-V) = 0.07 (Schlafy&Finkbeiner, 2011 ApJ 737, 103).