GCN Circular 39744

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

Swift/BAT did not localize GRB 250316A onboard (T0: 2025-03-16T08:59:57.690 UTC, Fermi GCN 39741

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 16.384 s analysis time bin starting at T0 + 8.192 s with a sqrt(TS) of 17.7. An arcminute localization is found with DeltaLLHOut of 72.75 and a DeltaLLHPeak of 42.6.

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 = 219.63, 14.886 deg which is

RA(J2000) = 14h 38m 31.20s

Dec(J2000) = 14d 53’ 09.60″

with an estimated uncertainty of 5 arcmin radius.

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

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

https://guano.swift.psu.edu/trigger_report?id=763808432

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/