TITLE:   GCN CIRCULAR
NUMBER:  39409
SUBJECT: GRB 250221A: Swift/UVOT Detection
DATE:    25/02/21 15:27:39 GMT
FROM:    Sam Shilling at Lancaster University <shilling.sam@gmail.com>

S.P.R. Shilling (Lancaster U.), A.A. Breeveld (UCL-MSSL)
and R. Caputo (NASA/GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250221A
107 s after the BAT trigger (Caputo et al., GCN Circ. 39396).
A source consistent with the enhanced XRT position
(Beardmore et al., GCN Circ. 39404) is detected in the initial UVOT exposures
and appears to be fading. An optical candidate in this field is
also detected by Watson et al., (GCN Circ. 39397) and by
Melandri et al., (GCN Circ. 39406).

Preliminary detections and 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: 

Filter         T_start(s)   T_stop(s)      Exp(s)           Mag

white              127          277	      147	    17.52 +/- 0.04
white		   620		639	       19	    18.41 +/- 0.17
v                  107          118	       10	    17.10 +/- 0.32
v		  4484	       4683	      197	    >18.8
b                  595          615            19	    >18.7  
u                  340          590           245	    17.78 +/- 0.07           
w1                4894         5041           144	    >19.0  
m2                4689	       4889	      197	    >18.6                            
w2                4279	       4479	      197	    >18.7         

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.332 in the direction of the burst
(Schlegel et al. 1998).