TITLE:   GCN CIRCULAR
NUMBER:  29885
SUBJECT: GRB 210421A: Swift/UVOT Upper Limits
DATE:    21/04/22 18:53:07 GMT
FROM:    Samantha Oates at MSSL  <samantha.oates@alumni.ucl.ac.uk>

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
96 s after the BAT trigger (Moss et al., GCN Circ. 29849).
No optical afterglow consistent with the XRT position
(Evans et al., GCN Circ. 29865).
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              96           246          147         >20.4
white  	             96          1912          191         >20.6
v      	         1608          1800           39         >17.9
b      	         1534          1899           58         >19.2
u_FC                  308           484          173         >19.5
u      	           308          1874          211         >19.6
uvw1   	         1658          1850           39         >18.1
uvm2   	         1633          1825           39         >17.9
uvw2   	         1584          1776           39         >18.3

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).