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GCN Circular 33315

Subject
GRB 230209B: Swift/BAT-GUANO detection outside the coded FOV
Date
2023-02-11T00:37:33Z (a year ago)
From
Aaron Tohuvavohu at U Toronto <aaron.tohu@gmail.com>
Gayathri Raman (PSU), Jamie A. Kennea (PSU), James DeLaunay
(UAlabama), Aaron Tohuvavohu (U Toronto) report:

Swift/BAT did not localize GRB 230209B onboard (T0:
2023-02-09T22:32:36 UTC, Fermi/GBM GCN 33310, INTEGRAL trig 10198).

The Fermi and INTEGRAL 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 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 2022, ApJ,
941, 169), detects the burst with a sqrt(TS) of 42.1 in a 2.048 s
analysis time bin.
The burst duration as seen by BAT is ~60 s.

NITRATES results indicate a burst coming from outside the FOV, with
DeltaLLHOut of -53.
The NITRATES best fit OFOV position is consistent with the Fermi/GBM
localization (GCN 33310).

See Section 9.1 and Figures 10 and 17 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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