TITLE:   GCN CIRCULAR
NUMBER:  43271
SUBJECT: GRB 251230A: Swift/UVOT detection
DATE:    25/12/30 13:45:04 GMT
FROM:    Alice Breeveld at MSSL-UCL <a.breeveld@ucl.ac.uk>

A.A. Breeveld (UCL-MSSL), N.P.M. Kuin (UCL-MSSL), S. Lanava (PSU) and M. De Pasquale (U. Messina) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 251230A 76s after the BAT trigger (Lanava et al., GCN Circ. 43252).
A bright, fading source consistent with the enhanced XRT position (Osborne et al., GCN Circ. 43263) and optical detections (Lanava et al., GCN 43252; Li et al., GCN 43255; Busmann et al., GCN 43258; Masi et al., GCN 43260; Ghosh et al., GCN 43262; Ma et al., GCN 43265, He et al., GCN 43269) is detected in the initial UVOT exposures in the white, v, b, u and uvw1 filters.

The burst is also detected in the UV grism and, while there is some contamination from other zeroth orders, we obtain a tentative redshift of z=1.73.

The preliminary UVOT position is:
    RA  (J2000) =  11:59:35.41 = 179.89756 (deg.)
    Dec (J2000) = +11:26:19.2  =  11.43867 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).

Preliminary detections using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the finding chart (FC) and other early settled exposures are:

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

white_FC            76          226          147         13.61 ± 0.03
white              868         1017          286         15.63 ± 0.02
v                  619          638           19         14.69 ± 0.07
b                  545          564           20         14.78 ± 0.05
u_FC               288          538          246         13.77 ± 0.02
uvw1               668          687           19         15.2  ± 0.1
uvm2               643         1775          136         >18.8
uvw2               594          614           19         >17.7

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.024 in the direction of the burst (Schlegel et al. 1998).