TITLE:   GCN CIRCULAR
NUMBER:  34873
SUBJECT: GRB 231018A: Swift/BAT-GUANO detection of a long burst outside the coded FOV
DATE:    23/10/23 18:21:25 GMT
FROM:    GAYATHRI RAMAN at PSU <gzr5209@psu.edu>

Tyler Parsotan (NASA GSFC), Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU), Samuele Ronchini (PSU), James DeLaunay (U Alabama), Jamie A. Kennea (PSU)  report: 

Swift/BAT did not localize GRB 231018A onboard (T0: 2023-10-18T12:30:10.66 UTC, Fermi trig 719325015-GCN#34826, CALET trigger 1381667434-GCN#34849) 

The Fermi 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 109.13 in a 16 s analysis time bin, starting at T0 - 12 s. The burst has a multi-peak structure with a duration of ~30 s in the BAT rates curve.

NITRATES results are consistent with a burst coming from outside the FOV, with DeltaLLHOut of -578.9.

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/