TITLE:   GCN CIRCULAR
NUMBER:  40009
SUBJECT: GRB 250327B: Swift/UVOT Detection
DATE:    25/04/02 13:32:05 GMT
FROM:    Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>

M.H. Siegel (PSU) reports on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250327B 1753 s after the SVOM trigger sb25032706 (GCN Circ. 39888).  A fading source consistent with the XRT position (Page et al., GCN Circ. 39895) and the optical counterpart (Moskvitin et al., GCN Circ. 39889; Xin et al., GCN Circ. 39890; O’Neill et al., GCN Circ. 39891; Lian et al., GCN Circ. 39894; Shrestha  et al., GCN Circ. 39896; Ducoin et al., GCN Circ. 39897; Perley & Bochenek; GCN Circ. 39902; Kumar et al., GCN Circ. 39904; Dennefeld et al., GCN circ. 39905; Pankov et al., GCN circ. 39906) is detected in the initial UVOT exposures.

Preliminary detections and 3-sigma upper limits for the early exposures are:

Filter		T_start(s)	T_stop(s)	Exp(s)		Mag(AB)
u                 1753           2505             740          19.78+/-0.18
u                91894         113326            2601         >21.28

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.018 in the direction of the burst (Schlegel et al. 1998).