TITLE: GCN CIRCULAR NUMBER: 35411 SUBJECT: GRB 231220A: Swift-XRT follow-up observations DATE: 23/12/21 15:25:45 GMT FROM: Phil Evans at U of Leicester pae9@star.le.ac.uk

C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), S. Dichiara (PSU), J.A. Kennea (PSU), A. Tohuvavohu (U. Toronto), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), R. Brivio (INAF-OAB) and P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team:

Swift-XRT has performed follow-up observations of the GUANO-detected burst GRB 231220A, collecting 3.6 ks of Photon Counting (PC) mode data between T0+27.0 ks and T0+39.5 ks.

No candidate counterpart was detected, with an upper limit of 2.2×10-3 ct s-1 at the best BAT position (GCN 35409).

One uncatalogued X-ray source was flagged as spurious after manual inspection.

The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021638.

This circular is an official product of the Swift-XRT team.

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