TITLE: GCN CIRCULAR NUMBER: 19931 SUBJECT: GRB 160917A: Swift/UVOT Detection DATE: 16/09/17 23:54:38 GMT FROM: Paul Kuin at MSSL N. P. M. Kuin (UCL-MSSL) and J. L. Racusin (NASA/GSFC) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 160917A 100 s after the BAT trigger (Racusin et al., GCN Circ. 19926). The refined XRT position (Evans et al. GCN Circ. 19927) is close to a bright star. However, a faint GRB candidate has been found at an offset of 6.5 arcsec to the south east from the bright star in the u exposures. The UVOT candidate position is: RA, Dec = 295.66687, 46.40368 (J2000) which is : RA = 19:42:40.049 Dec = +46:24:13.25 with an estimated accuracy of 1.5 arcsec. 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 100 224 123 >21.01 b 4222 4422 100 >20.64 u 4017 5545 146 20.47 +- 0.36 v 3402 5038 200 >19.77 uvw1 3812 5448 200 >20.16 uvm2 3606 5242 100 >19.97 uvw2 4633 4833 200 >19.70 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.16 in the direction of the burst (Schlegel et al. 1998).