GCN Circular 40600

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

Swift/BAT did not localize GRB 250601A onboard (T0: 2025-06-01T17:29:35.54 UTC, Fermi GCN 40596

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 standard imaging, with an SNR = 8.7. The duration of the burst is ~1 sec.

The BAT position is

RA, Dec = 90.0709, -57.2892 deg which is

RA(J2000) = 06h 00m 17.02s

Dec(J2000) = -57d 17’ 21.1″

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=770491811

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/