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GCN Circular 34661

Subject
GRB 230908A: Swift/BAT-GUANO detection of a long burst outside the coded FOV
Date
2023-09-11T16:08:54Z (9 months ago)
From
GAYATHRI RAMAN at PSU <gzr5209@psu.edu>
Via
Web form
Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU),  Jamie A. Kennea (PSU), James DeLaunay (U Alabama),  Samuele Ronchini (PSU), Tyler Parsotan (NASA GSFC) report: 

Swift/BAT did not localize GRB 230908A onboard (T0: 2023-09-08T16:41:17.84 UTC, CALET detection: trigger no. 1378226327, AstroSat detection: GCN 34645, GRBAlpha detection: GCN 34647) 

The CALET 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 90 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 76.33 in a 8.192 s analysis time bin, starting at T0 + 2.0482 s. 

NITRATES results are consistent with a burst coming from outside the FOV, with DeltaLLHOut of -134.81.

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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