GCN Circular 38900

Swift/BAT did not localize GRB 250109A onboard (T0: 2025-01-09T06:17:08.60 UTC, Fermi trig 758096233)

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), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 12.1 in a 16.384 s analysis time bin, starting at T0 - 8.192 s.

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. 2025. in prep)

The 90% credible area is 0.06 deg2 and the 50% credible area is ~0.01 deg2. The integrated probability inside the coded field of view is 98%.

55% of the NITRATES localization probability is contained within a 0.1 deg radius circle around the maximum probability position. The majority of the remaining probability is contained within similarly sized peaks of probability across the BAT coded field of view.

The maximum probability position of the NITRATES skymap is,

RA, Dec = 88.806, -12.458 deg which is

RA(J2000) = 05h 55m 13.4s

Dec(J2000) = -12d 27′ 28.8″

The NITRATES skymap is consistent with the Fermi localization reported in the final position notice (GCN 38873

). It is also consistent with the position of the X-ray transient EP250109a (GCN , GCN ), with the position lying on the 0.28 credible region contour. The spatial and temporal coincidence of GRB 250109A and EP250109a make it very likely that they originate from the same cosmic event.

A plot of the probability skymap can be viewed here:

skymap_plot

The probability skymap file can be downloaded from the link here

skymap_fits_file

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=758096263

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/