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GCN Circular 31096

Subject
GRB211118A: Swift/BAT-GUANO detection outside the coded FOV
Date
2021-11-19T15:52:44Z (3 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 trigger on GRB 211118A (T0: 2021-11-18 23:38:14 UT,
Fermi Trigger: 658971499, INTEGRAL/SPI-ACS trigger 9523).

The Fermi + INTEGRAL triggers, 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,
arXiv:2111.01769), detects the burst with a sqrt(TS) of 27.5 in a
1.024 s analysis time bin.
The estimated T90 in the detector is 3.0 +/- 0.05 s (15-350 keV)

NITRATES prefers an origin for this burst coming from outside the
coded FOV, with DeltaLLHOut of 2.7.

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

An out of FOV origin is consistent with the Fermi/GBM localization.

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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