GCN Circular 38308

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

The LIGO/Virgo/KAGRA detection of GW candidate S241125n (GCN 38305

), reported 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.

Using the NITRATES analysis (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find a candidate counterpart with sqrt(TS) of 7.41 in a 0.512 analysis time bin, starting at T0+11.2641 s.

The GRB candidate False Alarm Rate (FAR) is 3.74E-4 Hz. The joint GW-GRB FAR, combining the spatial and temporal information of both signals and correcting for trials, is 2.581E-9 Hz, or 1 every 12 years. This candidate has been submitted to the RAVEN pipeline (Urban 2016, Piotrzkowski 2022) for assessment, the joint FAR calculation above is computed by RAVEN.

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 7694 deg2 and the 50% credible area is 0.02 deg2. The integrated probability inside BAT the coded field of view is ~63%. Over half of the credible region is contained within a single ~arcminute position.

The BAT position is RA, Dec = 58.079, +69.689 deg which is RA(J2000) = 03h 52m 18.96s Dec(J2000) = +69d 41’ 20.4″ with an estimated uncertainty of 5 arcmin (50% containment).

Swift has already initiated TOO followup of this position with XRT and UVOT. Results will be reported in future circulars. We encourage followup by other, more sensitive, facilities.

The remainder of the localization posterior is spread over the sky.

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

A combined GW+GRB skymap can be found here: https://doi.org/10.5281/zenodo.14218311

More details about this burst can be found on the trigger report page here https://guano.swift.psu.edu/trigger_report?id=754189311