TITLE: GCN CIRCULAR NUMBER: 35409 SUBJECT: GRB 231220A: Swift/BAT-GUANO arcminute localization of a long burst DATE: 23/12/20 22:09:23 GMT FROM: Samuele Ronchini at PSU sjs8171@psu.edu

Samuele Ronchini (PSU), James DeLaunay (PSU), Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report:

Swift/BAT did not localize GRB 231220A onboard (T0: 2023-12-20T16:10:17.4 UTC, Fermi/GBM GCN 35407).

The Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1).

Upon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground.

The BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), detects the burst in a 4.096 s analysis time bin with a sqrt(TS) of 28.32, starting at T0 - 1.02 s. An arcminute localization is found with DeltaLLHOut of 48.29 and a DeltaLLHPeak of 49.13.

See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.

The BAT position is RA, Dec = 27.321, 77.897 deg which is RA(J2000) = 01h 49m 17.04s Dec(J2000) = +77d 53′ 49.2″ with an estimated uncertainty of 5 arcmin radius.

XRT and UVOT follow-up has been requested. Results of follow-up observations will be reported in future circulars.

GUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches.

A live reporting of Swift/BAT event data recovered by GUANO can be found at: https://www.swift.psu.edu/guano/

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