GCN Circular 35948

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

Swift/BAT did not localize GRB 240317B onboard (T0: 2024-03-17T10:54:24.00 UTC, Fermi/GBM trigger 732365669).

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 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 16.384 s analysis time bin with a sqrt(TS) of 49.27.
An arcminute localization is found with DeltaLLHOut of 385 and a DeltaLLHPeak of 501.

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 = 272.18, +39.182 deg which is
   RA(J2000)  = 18h 08m 43.2s
   Dec(J2000) = +39d 10′ 55.2″
with an estimated uncertainty of 15 arcmin radius, due to star tracker loss-of-lock.

Follow-up observations from other instruments are requested.
XRT and UVOT will not follow-up until March 18 due to planned observatory downtime.

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/