GCN Circular 37654

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 240930B onboard (T0: 2024-09-30 17:18:02.12 UTC, Fermi trig 749409487, possibly associated with EP240930a GCN 37648

The GECAM 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 18.7 in a 4.096 s analysis time bin, starting at T0 + 1.024 s.

With a DeltaLLHOut of 0.4, NITRATES results are able to constrain the position of this burst to most likely be outside the coded field of view.

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

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. 2024. in prep)

The 90% credible area is 2,566 deg2 and the 50% credible area is 491 deg2. The integrated probability inside the coded field of view is ~0%.

The localization of EP240930a is consistent with GRB 240930B and is fully within the 50% contour.

A plot of the probability skymap can be viewed here, https://guano.swift.psu.edu/trigger_report?id=749409516/#:~:text=Probability%20Skymap

The probability skymap file can be downloaded from the link here, https://guano.swift.psu.edu/files/749409516/0_n_PROBMAP

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=749409516

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/