TITLE: GCN CIRCULAR NUMBER: 11115 SUBJECT: GRB 100816A: Swift UVOT Observations DATE: 10/08/16 21:34:49 GMT FROM: Samantha Oates at MSSL S. R. Oates (MSSL-UCL) reports on behalf of the Swift/UVOT team: The Swift/UVOT began observing the field of GRB 100816A 91s after the BAT trigger (Oates et al., GCN Circ. 11102). We detect the optical afterglow significantly in the white and u filters and marginally in the v, b and uvw1 filters. The best UVOT position was reported in Oates et al. (GCN Circ. 11102), and is consistent with the XRT enhanced position (Evans et al., GCN Circ 11110) and with the optical afterglow position reported by TNG (Antonelli et al., GCN Circ 11104). Preliminary magnitudes and the 3 sigma upper limits are reported below for individual white and u images and summed images for the v, b, uvw1, uvm2 and uvw2 filters. Filter T_start(s) T_stop(s) Exposure Mag/3SigUL ########################################################### white (FC) 91 241 147 17.01 +/- 0.05 white 586 606 19 18.90 +/- 0.19 v 3991 5626 393 21.56 +/- 0.94 b 4811 6393 341 21.07 +/- 0.35 u (FC) 305 555 246 18.02 +/- 0.08 u 4606 4806 197 20.39 +/- 0.35 uvw1 4401 6036 393 20.98 +/- 0.44 uvm2 4196 34516 1279 > 21.42 uvw2 3787 28671 2475 > 22.19 ########################################################### The above magnitudes are not corrected for the Galactic extinction corresponding to a reddening of E_{B-V} = 0.09 mag (Schlegel et al., 1998, ApJS, 500, 525). The photometry is on the UVOT photometric system described in Poole et al. (2008, MNRAS, 383,627).