GCN Circular 43114

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

Swift/BAT did not localize GRB 251215A onboard (T0: 2025-12-15T04:21:21.57 UTC, Fermi GCN 43104

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 position is found with the newly developed pipeline BAT-GLIMPSE: Gamma-ray Localization using Imaging and Mosaic techniques for Pointing and Slew Epochs (Ronchini et. al, in prep). The pipeline makes use of the tools from BatAnalysis (Parsotan et al. 2025). The source was detected with an SNR = 6.5 in a 16.384 s analysis time bin starting at t0 - 4.096 s.

Independently, the BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), confirms the detection of the burst in a 16.384 s analysis time bin starting at T0 - 4.096 s with a sqrt(TS) of 14.6. An arcminute localization is found with DeltaLLHOut of 63.97 and a DeltaLLHPeak of 36.66. 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 NITRATES and BAT-GLIMPSE positions are compatible.

Moreover, at T0: 2025-12-15T04:23:36.28 UTC, also a CALET trigger (trig number 1449807236) activated GUANO. A signal is found by BAT-GLIMPSE during Swift slew using a mosaic technique. The source is found with an SNR = 27.1. The position is compatible with the one found by BAT-GLIMPSE and NITRATES for the Fermi-GBM trigger. Hence we conclude that the two signals are associated to the same GRB.

The BAT position is:

RA, Dec = 122.6526, -36.7395 deg which is

RA(J2000) = 08h 10m 36.62s

Dec(J2000) = -36d 44’ 22.2″

with an estimated uncertainty of 3 arcmin radius.

More details about this burst can be found on the trigger report page here: https://guano.swift.psu.edu/trigger_report?id=787465318

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/