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GCN Circular 34683

Subject
GRB 230913A: Swift/BAT-GUANO arcminute localization of a burst
Date
2023-09-13T13:55:53Z (7 months ago)
From
Samuele Ronchini at PSU <sjs8171@psu.edu>
Via
Web form
Samuele Ronchini (PSU), Jamie A. Kennea (PSU), James DeLaunay (U Alabama), Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU), Tyler Parsotan (NASA GSFC)  report:

Swift/BAT did not localize GRB 230913A onboard (T0: 2023-09-13T07:56:12.16 UTC, Fermi/GBM GCN 34680, CALET trigger 1378626813).

The Fermi 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 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 in a 8.192 s analysis time bin with a sqrt(TS) of 19.7.
An arcminute localization is found with DeltaLLHOut of 64.99 and a DeltaLLHPeak of 60.07.

See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretations of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.

The BAT position is
RA, Dec = 267.349, +74.484 deg (J2000) which is
RA (J2000)      17h 49m 23.8s
Dec (J2000)    74d 29m 2.4s
with an estimated uncertainty of 5 arcmin radius.

No XRT and UVOT follow-up can be done due to observational constraints. 
We strongly encourage follow-up from other telescopes.

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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