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GCN Circular 31824

Subject
GRB 220403C (or A): Swift/BAT-GUANO detection outside the coded FOV
Date
2022-04-04T01:46:45Z (2 years ago)
From
Aaron Tohuvavohu at U Toronto <aaron.tohu@gmail.com>
Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU), James DeLaunay
(UAlabama), Jamie A. Kennea (PSU), report:

Swift/BAT did not localize GRB 220403C onboard (T0: 2022-04-03T10:13:50
 UTC, INTEGRAL SPI/ACS trig #9822 CALET trig #1333015963).

The INTEGRAL and CALET notices, 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 224.4 in a
8.192 s analysis time bin.
The burst episode as seen by BAT is ~15s long.

NITRATES results strongly indicate a burst coming from outside the
coded FoV, with DeltaLLHOut of -1614.

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

GUANO was also triggered by notice of GRB 220403A, detected by
Fermi/GBM with T0: 2022-04-03T10:10:02 UTC (GCN 31818).
GUANO retrieved 200s of event data from [-50,+150] seconds around the
time of the burst. This Fermi/GBM trigger was 228 seconds before the
emission episode reported above.

This burst is not detected by NITRATES running on the BAT-GUANO data.

The peak flux of the first emission episode in Fermi/GBM is
substantially weaker compared to that seen for the second in BAT, and
its non-detection in the BAT data is not necessarily indicative of a
different progenitor object, especially considering the OFOV origin.

GRB 220403A could be a weak precursor episode to GRB 220403C, produced
by the same object. Enhanced localizations for both bursts will be
required to determine their relationship, if any.

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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