TITLE: GCN CIRCULAR NUMBER: 35440 SUBJECT: GRB 231230A: Swift/UVOT Upper Limits DATE: 23/12/30 11:25:57 GMT FROM: Paul Kuin at MSSL npkuin@gmail.com

N. P. M. Kuin (UCL-MSSL) and M. J. Moss (GSFC) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 231230A 1511 s after the BAT trigger (Moss et al., GCN Circ. 35436). No optical afterglow consistent with the XRT position 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

white 1511 1705 39 >18.9 b 1661 1681 19 >18.5 w1 1612 1632 19 >18.0

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.014 in the direction of the burst (Schlegel et al. 1998).

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