TITLE:   GCN CIRCULAR
NUMBER:  39443
SUBJECT: LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO trigger with ID 762004910: Identification of a GW compact binary merger candidate possibly associated with a sub-threshold Swift/BAT-GUANO trigger
DATE:    25/02/24 03:12:15 GMT
FROM:    minghuidu1993@gmail.com

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT-GUANO team report:

We identified the compact binary merger candidate S250223dk during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at T0 = 2025-02-23 12:01:15.360 UTC (GPS time: 1424347293.360). The candidate was found by the GstLAL [1] analysis pipeline.

S250223dk is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.4e-05 Hz, or about one in 19 hours. The event's properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250223dk

The classification of the GW signal, in order of descending probability, is Terrestrial (92%), BBH (8%), NSBH (<1%), or BNS (<1%).

Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

Five GW-only sky maps are available at this time and can be retrieved from the GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [3], distributed via GCN notice about a minute after the candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 5 minutes after the candidate event time.
 * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 12 hours after the candidate event time.
 * bayestar.multiorder.fits,3, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 12 hours after the candidate event time.
 * bayestar.multiorder.fits,4, an initial localization generated by BAYESTAR [3], distributed via GCN notice about 13 hours after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,4. For the bayestar.multiorder.fits,4 sky map, the 90% credible region is 3203 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 6033 +/- 1778 Mpc (a posteriori mean +/- standard deviation).

The LVK notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; [4]). 

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 [-45,+45] seconds around the time of the GW alert. All the requested event mode data was delivered to the ground. 

The BAT likelihood search, NITRATES [5], performed on the temporal window [T0-20 s, T0+20 s], detects a burst candidate with a sqrt(TS) of 8.0 in a 8.192 s analysis time bin, starting at T0 - 10.240 s 

The 90% credible area is 2666 deg2 and the 50% credible area is <1 deg2.
The integrated probability inside the coded field of view is 83%. 

The joint LVK-Swift/BAT localization probability map peaks at 
RA = 85.341 deg,
Dec =  -47.554 deg.
A circle with a radius of 10 arcmin around this position contains 74% of the integrated joint probability.

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.

A plot of the Swift/BAT probability skymap can be viewed here:

[https://guano.swift.psu.edu/trigger_report?id=762004910/#:~:text=Probability%20Skymap](https://guano.swift.psu.edu/trigger_report?id=762004910/#:~:text=Probability%20Skymap)

The Swift/BAT probability skymap file can be downloaded from the link here:

[https://guano.swift.psu.edu/files/762004910/0_n_PROBMAP](https://guano.swift.psu.edu/files/762004910/0_n_PROBMAP)

Instructions on how to read and manipulate this map can be found here:

[https://guano.swift.psu.edu/documentation](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=762004910](https://guano.swift.psu.edu/trigger_report?id=762004910)


A search performed by the RAVEN pipeline [6] found a temporal coincidence between S250223dk and a sub-threshold Swift/BAT trigger with ID 762004910. The GRB trigger time is 10.24 seconds before the GW candidate event. The estimated joint false alarm rate for the coincidence using just timing info is 1.9e-07 Hz, or about one in a month. The GRB candidate was found during a joint targeted search between the LIGO/Virgo/KAGRA collaboration and Swift/BAT-GUANO, and has a false alarm rate of 7.4e-05 Hz, or about one in 3 hours.Combined sky maps are also available:
 * combined-ext.multiorder.fits,0, an initial localization, distributed via GCN notice about 12 hours after the candidate event time.
 * combined-ext.multiorder.fits,1, an initial localization, distributed via GCN notice about 12 hours after the candidate event time.
 * combined-ext.multiorder.fits,2, an initial localization, distributed via GCN notice about 13 hours after the candidate event time.

For the combined-ext.multiorder.fits,2 sky map, the 90% credible region is <1 deg2. The joint localization is dominated by the Swift/BAT-GUANO candidate. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence is 5.1e-09 Hz, or about one in 6 years.

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/.

 [1] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625
 
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 
 [3] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013
 
 [4] Tohuvavohu et al. ApJ, 900, 1 (2020)
 
 [5] DeLaunay & Tohuvavohu, ApJ, 941, 169 (2022)
 
 [6] Urban, A. L. 2016, Ph.D. Thesis https://dc.uwm.edu/etd/1218 and Piotrzkowski, B. J. 2022, Ph.D. Thesis https://dc.uwm.edu/etd/3060