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GCN Circular 31338

Subject
GRB 211229C: Swift/BAT-GUANO detection likely outside the coded FOV
Date
2021-12-30T01:23:12Z (2 years ago)
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/
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