GCN Circular 42732

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

Swift/BAT did not localize GRB 251117A onboard (T0: 2025-11-17T12:19:52.14 UTC, Fermi Trig 785074797, SVOM/GRM GCN 42731

)

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), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 10.0 in a 1.024 s analysis time bin, starting at T0 - 0.256 s.

Using the NITRATES analysis, parameter estimation was performed to obtain the localization of this burst in the form of a HEALPIX Multi-Order Coverage (MOC) skymap. This localization accounts for both statistical and systematic errors. More details in the creation and calibration of these maps will soon be published (DeLaunay et al. 2025. in prep)

The 90% credible area is 12,010 deg2 and the 50% credible area is 4,336 deg2. The integrated probability inside the coded field of view is 1%.

The NITRATES skymap is consistent with the Ferm/GBM localization reported in the final position notice (GCN 42719

). The combined Fermi/GBM+NITRATES 90% credible area is 439 deg2 and the 50% credible area is 120 deg2.

The NITRATES and combined localizations are consistent with the Fermi/LAT localization (GCN 42729

)

A plot of the probability skymap can be viewed here:

skymap_plot

The probability skymap and joint skymap files can be downloaded from the links here #The probability skymap file can be downloaded from the link here

skymap_fits_file

joint_skymap_fits_file

Instructions on how to read and manipulate this map can be found here:

https://guano.swift.psu.edu/documentation

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

https://guano.swift.psu.edu/trigger_report?id=785074828

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/