TITLE:   GCN CIRCULAR
NUMBER:  40002
SUBJECT: GRB 250331B: Swift/UVOT Upper Limits
DATE:    25/04/01 13:04:29 GMT
FROM:    Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>

M. H. Siegel (PSU) and S. B. Cenko (GSFC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250331B
317 s after the BAT trigger (Cenko et al., GCN Circ. 39958).
No optical afterglow consistent with the XRT position (Osborne et al., GCN
Circ. 39974) 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           317          467          147         >19.5
white              317         1020          334         >19.9
v                  474         1070           78         >18.3
b                  572          764           39         >18.3
u                  547         1144           58         >18.6
w1                 523         1120           58         >18.0
m2                 670          690           19         >17.7
w2                 621         1046           58         >18.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.979 in the direction of the burst
(Schlegel et al. 1998).