TITLE:   GCN CIRCULAR
NUMBER:  40003
SUBJECT: GRB 250331C: Swift/UVOT Upper Limits
DATE:    25/04/01 13:06:09 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 250331C
140 s after the BAT trigger (Cenko et al., GCN Circ. 39959).
No optical afterglow consistent with the XRT position (Beardmore et al., 
GCN Circ. 39977) 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           140          289          147         >21.1
u_FC               297          547          246         >19.9
white              140         1717          411         >22.3
v                  628         1767          136         >19.1
b                  553         1863          133         >20.1
u                  297         1841          363         >20.1
w1                 676         1817          136         >19.1
m2                 652         1792          136         >19.8
w2                 603         1743          136         >20.0

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.921 in the direction of the burst
(Schlegel et al. 1998).