TITLE: GCN CIRCULAR NUMBER: 41264 SUBJECT: GRB 250806A: Swift/UVOT U-band detection DATE: 25/08/07 11:15:10 GMT FROM: Alice Breeveld at MSSL-UCL a.breeveld@ucl.ac.uk

A.A. Breeveld (UCL-MSSL) and Paul Kuin (UCL-MSSL) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of the SVOM/ECLAIRS detected burst GRB 250806A (Xie et al., GCN Circ. 41243) 354 s after the SVOM trigger. The X-ray afterglow reported by Campana et al. (GCN Circ. 41249) and Liang et al. (GCN Circ. 41260) is detected in the initial U-band exposure, but has faded in the later white band exposures.

Preliminary magnitude and 3-sigma upper limit using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the initial exposures is:

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

u 354 1370 1000 20.5 ± 0.3 white 63123 64122 975 > 21.7

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V)=0.044 in the direction of the burst (Schlegel et al. 1998).

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