TITLE:   GCN CIRCULAR
NUMBER:  41435
SUBJECT: GRB 250818B: Swift/UVOT Detection
DATE:    25/08/20 04:23:17 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 250818B 1.7 ks after the SVOM/ECLAIRs trigger (Wang et al., GCN Circ. 41405). An uncatalogued source consistent with the XRT position (Farro et al., GCN Circ. 41407) and the optical transient (Kumar et al., GCN Circ. 41406; Yao et al., GCN Circ. 41409; Zheng et al., GCN Circ. 41417; Bendtsen et al., GCN Circ. 41426; Moskvitin et al., GCN Circ. 41428; An et al., GCN Circ. 41430; Kumar et al., GCN Circ. 41431) is detected in the UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  03:04:13.45 =  46.05602 (deg.)
    Dec (J2000) = -03:07:31.2  =  -3.12532 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).

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

u                 1723         6610         1544      18.25+/-0.06

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.063 in the direction of the burst
(Schlegel et al. 1998).