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). View this GCN Circular online at https://gcn.nasa.gov/circulars/43271. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...