TITLE: GCN CIRCULAR NUMBER: 38309 SUBJECT: LIGO/Virgo/KAGRA S241125n: Updated Sky localization and Coincidence with External Event DATE: 24/11/25 18:38:57 GMT FROM: Brandon Piotrzkowski <brandon.piotrzkowski@ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration along with the Swift/BAT Collaboration 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 S241125n (GCN Circular 38305). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page: https://gracedb.ligo.org/superevents/S241125n Note that this skymap may be affected by the noise transient (glitch) mentioned in the original circular. For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 1957 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4474 +/- 1675 Mpc (a posteriori mean +/- standard deviation). A search performed by the RAVEN pipeline [2] found a temporal coincidence between S241125n and a sub-threshold Swift/BAT trigger with ID 754189311 (DeLaunay et al., GCN Circular 38308). The GRB trigger time is 0.2 seconds after the GW candidate event. The estimated joint false alarm rate for the coincidence using just timing info is 1.5e-10 Hz, or about one in 1e2 years. 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 0.00037 Hz, or about one in 44 minutes. A combined sky map is also available: * combined-ext.multiorder.fits,0, an updated localization, distributed via GCN notice about 17 hours after the candidate event time. For the combined-ext.multiorder.fits,0 sky map, the 90% credible region is 873 deg2. The joint localization is dominated by the Swift/BAT candidate, which was identified with right ascension, declination of 58.079, +69.689 deg. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence is 1.2e-10 Hz, or about one in 1e2 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] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040 [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 View this GCN Circular online at https://gcn.nasa.gov/circulars/38309. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...