GCN Circular 43033

James DeLaunay (PSU), Aaron Tohuvavohu (Cosmic Frontier), Samuele Ronchini (GSSI), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC), Maia Williams (Northwestern)  report:

Swift/BAT did not localize GRB 251208B onboard (T0: 2025-12-08T10:17:17.29 UTC, Fermi Trig 786881842).

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 8.192 s analysis time bin starting at T0 - 2.048 s with a sqrt(TS) of 17.6.
An arcminute localization is found with DeltaLLHOut of 43 and a DeltaLLHPeak of 30.0.

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 = 228.453, 29.044 deg which is
   RA(J2000)  = 15h 13m 48.72s
   Dec(J2000) = 29d 02’ 38.4″
with an estimated uncertainty of 5 arcmin radius.

More details about this burst can be found on the trigger report page here:

https://guano.swift.psu.edu/trigger_report?id=786881874

XRT and UVOT follow-up has been requested.
Results of follow-up observations will be reported in future circulars.

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/