TITLE:   GCN CIRCULAR
NUMBER:  39814
SUBJECT: GRB 250321A: Swift/UVOT Upper Limits
DATE:    25/03/21 13:55:16 GMT
FROM:    Sam Shilling at Lancaster University <shilling.sam@gmail.com>

S. P. R. Shilling (Lancaster U.) and R. Gupta (NASA/GSFC) report
on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250321A
128 s after the BAT trigger (Gupta et al., GCN Circ. 39794).
No optical afterglow consistent with the XRT position
(Page et al., GCN Circ. 39798) 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           128          278          147         >19.9
white              128         4071          344         >20.6
v                 4282         4482          197         >18.9
b                 3667         5142          236         >20.0
u                  340         5097          232         >19.4
w1                4693         4892          197         >19.3
m2                4487         4687          197         >19.2
w2                4077         4277          197         >19.3

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 4.368 in the direction of the burst
(Schlegel et al. 1998).