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GCN Circular 34105

Subject
GRB 230628B: Swift/BAT-GUANO detection of a burst outside the coded FOV
Date
2023-06-28T16:42:07Z (a year ago)
From
Jimmy DeLaunay at University of Alabama <delauj2@gmail.com>
James DeLaunay (U Alabama), Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU), Samuele Ronchini (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report: 

Swift/BAT did not localize GRB 230628B onboard (T0: 2023-06-28T01:35:56.03 UTC, INTEGRAL trig 10298, Fermi trig. 709608965). 

The INTEGRAL and Fermi 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),  performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 14.54 in a 16.384 s analysis time bin, starting at T0.

NITRATES results are consistent with a burst coming from outside the FOV, with DeltaLLHOut of 0.78. 

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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