TITLE: GCN CIRCULAR NUMBER: 30572 SUBJECT: GRB 210731A: Swift/UVOT Detection of an optical afterglow DATE: 21/08/01 14:44:28 GMT FROM: Paul Kuin at MSSL N. P. M. Kuin (UCL-MSSL) and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift/UVOT team: The Swift/UVOT began settled observations of the field of GRB 210731A 210 s after the BAT trigger (Troja et al., GCN Circ. 30568). An optical afterglow consistent with the XRT position (D'Ai et al. GCN Circ. 30569) and discovered by Meerlicht (de Wet ert al, GCN Circ. 30570) is also detected in the initial UVOT exposures. Interestingly, the initial rise to peak is slow. Preliminary 3-sigma magnitudes using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 210 273 62 20.2 +/- 0.38 b 17547 18204 587 19.02 +/- 0.08 u 29863 30504 624 18.66 +/- 0.09 v 11787 12145 348 18.60 +/- 0.17 uvw1 28956 29857 886 19.33 +/- 0.14 uvw2 34767 35667 886 20.41 +/- 0.27 uvm2 23308 23846 530 19.40 +/- 0.21 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.092 in the direction of the burst (Schlegel et al. 1998).