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GCN Circular 31677

Subject
GRB 220304A: Swift/BAT-GUANO detection outside the coded FOV
Date
2022-03-04T16:15:57Z (2 years ago)
From
Aaron Tohuvavohu at U Toronto <aaron.tohu@gmail.com>
James DeLaunay (UAlabama), Aaron Tohuvavohu (U Toronto), Gayathri
Raman (PSU), Jamie A. Kennea (PSU), report:

Swift/BAT did not localize GRB 220304A onboard (T0: 2022-03-04T05:28:58
 UTC, Fermi/GBM GCN 31672).

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,
arXiv:2111.01769), detects the burst with a sqrt(TS) of 49 in a 16.384
s analysis time bin, the longest time bin of the search.
The duration of the burst is at least 30 seconds.

NITRATES results indicate a burst coming from outside the coded FOV,
with DeltaLLHOut of -55. The best fit location is consistent with the
Fermi/GBM RoboBA (GCN 31672), and BALROG (GCN 31674) localizations.

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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