TITLE:   GCN CIRCULAR
NUMBER:  38850
SUBJECT: GRB 250108B: Swift/UVOT Upper Limits
DATE:    25/01/08 14:32:24 GMT
FROM:    Alice Breeveld at MSSL-UCL <a.breeveld@ucl.ac.uk>

A. A. Breeveld (UCL-MSSL) and N. J. Klingler (GSFC/UMBC/CRESSTII) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 250108B 117 s after the BAT trigger (Klingler et al., GCN Circ. 38847).
No optical afterglow consistent with the enhanced XRT position (Osborne et al., GCN Circ. 38848) 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           117          267          147         >20.7
white              117          980          247         >21.2
u_FC               275          525          246         >20.2

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.012 in the direction of the burst (Schlegel et al. 1998).