TITLE: GCN CIRCULAR NUMBER: 35242 SUBJECT: GRB 231129C: Swift/BAT-GUANO localization of a burst outside the coded FOV DATE: 23/12/01 03:52:27 GMT FROM: Jimmy DeLaunay at Penn State delauj2@gmail.com

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

Swift/BAT did not localize GRB 231129C onboard (T0: 2023-11-29T19:10:15.02 UTC, CALET trig 1385320118 / GCN 35223 Fermi GBM trig 722460326 / GCN 35221, GECAM trig 238 / GCN 35231, MAXI-GSC GCN 35223, AstroSat GCN 35230, Glowbug GCN 35235, GRBAlpha GCN 35236, Fermi LAT GCN 35238)

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 90 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 92.6 in a 4.096 s analysis time bin, starting at T0 + 3.072 s.

NITRATES results are consistent with a burst coming from outside the FOV, with DeltaLLHOut of -477.

See Section 9.1 and Figures 10 and 17 in the NITRATES paper for brief descriptions and interpretation of sqrt(TS), DeltaLLHPeak, and DeltaLLHOut.

A preliminary localization of this GRB was performed, finding a best fit position of RA, Dec = 301.3, -80.4 deg With a roughly circular 90% error radius of 11.1 deg

Calibration of systematics for localizations outside the coded field of view is not yet complete.

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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