TITLE: GCN CIRCULAR NUMBER: 31338 SUBJECT: GRB 211229C: Swift/BAT-GUANO detection likely outside the coded FOV DATE: 21/12/30 01:23:12 GMT FROM: Aaron Tohuvavohu at U Toronto aaron.tohu@gmail.com

Gayathri Raman (PSU), James DeLaunay (UAlabama), Aaron Tohuvavohu (U Toronto), Jamie A. Kennea (PSU), report:

Swift/BAT did not localize GRB 211229C (T0: 2021-12-29T09:36:08 UTC,INTEGRAL/SPI-ACS 9580).

The INTEGRAL 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 90 seconds of BAT event-mode data from [-45,+45] 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, arXiv:2111.01769), detects the burst with a sqrt(TS) of 10.4 in a 4.096 s analysis time bin.

NITRATES results, independently, are ambiguous with respect to whether this burst originates from in or outside the BAT FOV, with a borderline DeltaLLHOut of 7 and no specific location in the FOV preferred.

Independent spectral or fluence measurements of this burst from other instruments could help determine the preferred origin.

See Section 9.1 and Figure 20 in the NITRATES paper for brief descriptions and interpretation of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.

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/