TITLE: GCN CIRCULAR NUMBER: 39499 SUBJECT: LIGO/Virgo/KAGRA S250223dk and Swift/BAT-GUANO ID 762004910: Updated Sky localization and Coincidence with External Event DATE: 25/02/26 18:26:53 GMT FROM: ethan.payne@ligo.org

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

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250223dk (GCN Circular 39443). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

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

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 18323 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 9589 +/- 6347 Mpc (a posteriori mean +/- standard deviation).

A search performed by the RAVEN pipeline [2] found a temporal coincidence between S250223dk and a sub-threshold Swift/BAT trigger with ID 762004910 (GCN circular 39443). 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. RAVEN has also identified an additional detection from Fermi GBM.

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. * combined-ext.multiorder.fits,3, an initial localization, distributed via GCN notice about 15 hours after the candidate event time. * combined-ext.multiorder.fits,5, an updated localization, distributed via GCN notice about 3 days after the candidate event time.

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 52% of the integrated joint probability.

For the combined-ext.multiorder.fits,5 sky map, the 90% credible region is 1 deg2. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence is 1.7e-08 Hz, or about one in 2 years. After considering trials factors, this means the joint FAR is larger than the ‘'high significance’' alert threshold and this joint event is no longer considered significant.

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] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. (2023) arXiv:2307.13380 [2] 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

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