TITLE: GCN CIRCULAR NUMBER: 30729 SUBJECT: GRB 210826A: Swift/UVOT Upper Limits DATE: 21/08/27 15:38:05 GMT FROM: Alice Breeveld at MSSL-UCL a.breeveld@ucl.ac.uk

A. A. Breeveld (UCL-MSSL) and T. Sbarrato (INAF-IASFPA) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210826A 36747 s after the LAT trigger (Ohno et al., GCN Circ. 30721). No new optical source consistent with the any of the 8 candidate XRT positions (D'Ai et al., GCN Circ. 30725) is detected in the initial UVOT exposures. Preliminary 3-sigma upper limits 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

v 36747 49716 4869 >20.9

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.032 in the direction of the burst (Schlegel et al. 1998).