GCN Circular 34084

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

Swift/BAT did not localize GRB 230625B onboard (T0: 2023-06-25T07:28:45.61 UTC, CALET trig. 1371713313). 

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 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 10.93 in a 16.384 s analysis time bin, starting at T0+4.096 s.

NITRATES results, independently, are ambiguous with respect to whether this burst originates from inside or outside the BAT coded FOV, with a DeltaLLHOut of 12.81 and DeltaLLHPeak of 2.84.

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/