TITLE: GCN CIRCULAR NUMBER: 39979 SUBJECT: GRB 250331D: Swift/BAT-GUANO arcminute localization of a burst during slew DATE: 25/03/31 17:36:04 GMT FROM: Samuele Ronchini at PSU sjs8171@psu.edu

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 250331D onboard (T0: 2025-03-31T05:49:17 UTC, Fermi GCN 39961).

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 burst occurred during a Swift slew. The imaging is performed using BatAnalysis tools (Parsotan et al. 2025 [https://arxiv.org/abs/2502.00278%5D(https://arxiv.org/abs/2502.00278)). A confident location is found for the burst with SNR of 11.6

The BAT position is

RA, Dec = 48.52543, 62.30596 deg which is

RA(J2000) = 03h 14m 06.10s

Dec(J2000) = +62d 18’ 21.5″

with an estimated uncertainty of 5 arcmin.

This position is consistent with the Ferm/GBM localization (GCN 39961).

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/

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