TITLE:   GCN CIRCULAR
NUMBER:  38866
SUBJECT: GRB 250108A: Swift/UVOT Upper Limits
DATE:    25/01/09 10:33:26 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 SVOM/ECLAIRs-detected burst GRB 250108A 102468 s after the SVOM trigger (Zheng et al., GCN Circ. 38845).
No optical afterglow consistent with either the Swift-XRT proposed X-ray counterpart (Ambrosi et al., GCN Circ. 38853) or the VT optical afterglow candidate (Qiu et al., GCN Circ. 38852), 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 initial exposures are:

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

w1              102468       109462         2632         >20.9

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).