TITLE: GCN CIRCULAR NUMBER: 19921 SUBJECT: GRB 160912A: Swift/UVOT Weak Afterglow Detection DATE: 16/09/13 16:10:01 GMT FROM: Frank Marshall at GSFC F. E. Marshall (NASA/GSFC) and J. K. Cannizzo (NASA/UMBC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 160912A 138 s after the BAT trigger (Cannizzo et al., GCN Circ. 19912). There is a weak detection of a source consistent with the XRT position (Beardmore et al., GCN Circ. 19915) and the optical afterglow positions (Tanvir and Levan, GCN Circ. 19918; Mazaeva et al., GCN Circ. 19919) in the initial UVOT exposures. There is little evidence for variability. The preliminary UVOT position is: RA (J2000) = 20:05:59.74 = 301.49893 (deg.) Dec (J2000) = +57:33:56.0 = 57.56555 (deg.) with an estimated uncertainty of 1.0 arcsec. (radius, 90% confidence). Preliminary detections and 2-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 138 8135 883 22.03 +/- 0.27 v 626 7110 411 20.60 +/- 0.44 b 552 7931 584 >22.14 u 296 7726 814 >21.92 w1 675 7520 588 >21.25 m2 650 7314 391 >20.75 w2 602 8215 484 >21.05 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.20 in the direction of the burst (Schlegel et al. 1998).