NOTE: uvot_fluxes_wget.pl vrs. 2020-11-03a fem at 210421d15h18m35s723112usUT
NOTE: Source region spec. is: 104.8756 +4.9216 4"
NOTE: Used archived XRT position of 104.87560   4.92160  4.00
NOTE: position reference is GCN_Notice

=========== Draft Upper Limit Circular ========

GRB 210421A: Swift/UVOT Upper Limits

S. R. Oates (U.Birmingham) and M. J. Moss (GWU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210421A
97 s after the BAT trigger (Moss et al., GCN Circ. 29849).
No optical afterglow consistent with the XRT position
*** POSITION REFERENCE is GCN_Notice ***
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 first
finding chart (FC) exposure and subsequent exposures are:

Filter         T_start(s)   T_stop(s)      Exp(s)         Mag

white_FC            97          246          147         >20.6
white               97          246          147         >20.6
u                  309          484          173         >20.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.318 in the direction of the burst
(Schlegel et al. 1998).