TITLE: GCN CIRCULAR NUMBER: 37604 SUBJECT: LIGO/Virgo/KAGRA S240925n: Identification of a GW compact binary merger candidate DATE: 24/09/25 01:53:06 GMT FROM: Chia-Hsuan Hsiung <sw56540@gmail.com> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S240925n during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2024-09-25 00:58:09.990 UTC (GPS time: 1411261107.990). The candidate was found by the cWB [1], cWB BBH [2], GstLAL [3], MBTA [4], and SPIIR [5] analysis pipelines. S240925n is an event of interest because its false alarm rate, as estimated by the online analysis, is 4e-20 Hz, or about one in 1e12 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S240925n The classification of the GW signal, in order of descending probability, is BBH (>99%), BNS (<1%), NSBH (<1%), or Terrestrial (<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%. [6] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [6] Both HasNS and HasRemnant consider the support of several neutron star equations of state for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 26%. Two 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 [7], distributed via GCN notice about 30 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [7], distributed via GCN notice about 5 minutes after the candidate event time. The preferred sky map at this time is bayestar.multiorder.fits,1. For the bayestar.multiorder.fits,1 sky map, the 90% credible region is 40 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 329 +/- 76 Mpc (a posteriori mean +/- standard deviation). 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] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004 [2] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [3] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625 [4] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913 [5] Chu et al. PRD 105, 024023 (2022) doi:10.1103/PhysRevD.105.024023 [6] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [7] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013 View this GCN Circular online at https://gcn.nasa.gov/circulars/37604. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...