TITLE:   GCN CIRCULAR
NUMBER:  36214
SUBJECT: GRB 240421B: Swift/UVOT Upper Limits
DATE:    24/04/21 19:04:49 GMT
FROM:    Paul Kuin at MSSL <npkuin@gmail.com>

N. P. M. Kuin (UCL-MSSL) and M. Williams (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 240421B
130s after the BAT trigger (Williams et al., GCN Circ. 36208).  No
optical afterglow consistent with the XRT position Beardmore et al.
(GCN Circ. 36213) 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           130          279          148         >20.6
u_FC               343          592          246         >20.2
white              130         1708          218         >20.9
v                  672         1764           78         >18.4
b                  598         1854           84         >19.8
u                  343         1827          339         >20.2
w1                 721         1813           78         >19.4
w2                 648         1568           78         >18.8

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.182 in the direction of the burst
(Schlegel et al. 1998).