GCN Circular 40796

James DeLaunay (PSU), Rachel Hamburg (USRA), Oindabi Mukherjee (USRA), Samuele Ronchini (PSU), Aaron Tohuvavohu (Caltech), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC), Maia Williams (PSU), Gayathri Raman (PSU) report:

Swift/BAT did not localize GRB 250620A onboard (T0: 2025-06-20T08:05:18.94 UTC, Fermi/GBM trig 772099523)

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 7.3 in a 0.512 s analysis time bin, starting at T0 + 8.96 s. This corresponds to a FAR ~ 5e-4 Hz.

This is ~9 s after the initial short spike detected by Fermi/GBM, but is temporally coincident with a separate pulse seen in the Fermi/GBM lightcurve.

The initial short spike is also detected by NITRATES, but very weakly with a sqrt(TS) of 6.8 (FAR ~ 2e-3 Hz) in a 0.256 s analysis time bin, starting at T0 - 0.128 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 15,094 deg2 and the 50% credible area is 5,333 deg2. The integrated probability inside the coded field of view is 1%.

The NITRATES skymap is very large but is consistent with the Fermi localization reported in the final position notice (GCN 40791

A plot of the probability skymap can be viewed here:

skymap_plot

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=772099554

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/