TITLE:   GCN CIRCULAR
NUMBER:  33673
SUBJECT: GRB 230420A: Swift/UVOT Upper Limits
DATE:    23/04/21 20:14:30 GMT
FROM:    Samantha Oates at University of Birmingham <samantha.oates@alumni.ucl.ac.uk>

S. R. Oates (U.Birmingham) and N. J. Klingler (GSFC/UMBC/CRESSTII)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 230420A
99 s after the BAT trigger (Klingler et al., GCN Circ. 33658).
No optical afterglow consistent with the XRT position
(Osborne et al., GCN Circ. 33661) 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            99          249          147         >21.1
u_FC               258          508          246         >20.5
white               99         1190          353         >21.7
v                  590         4885          255         >19.5
b                  514         1165           58         >19.9
u                  258         1141          285         >20.6
w1                 814         5109           52         >19.1
w2                4481         4681          197         >19.8

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.017 in the direction of the burst
(Schlegel et al. 1998).