TITLE: GCN CIRCULAR NUMBER: 39148 SUBJECT: LIGO/Virgo/KAGRA S250205bk: Identification of a GW compact binary merger candidate DATE: 25/02/05 11:12:59 GMT FROM: Lorenzo Asprea at INFN Torino <lorenzo.asprea@to.infn.it> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report: We identified the compact binary merger candidate S250205bk during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-02-05 10:35:41.575 UTC (GPS time: 1422786959.575). The candidate was found by the CWB-BBH [1], GstLAL [2], MBTA [3], PyCBC Live [4], and SPIIR [5] analysis pipelines. S250205bk is an event of interest because its false alarm rate, as estimated by the online analysis, is 9.5e-10 Hz, or about one in 33 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S250205bk The classification of the GW signal, in order of descending probability, is BBH (98%), NSBH (1%), Terrestrial (<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%. [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. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. 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 39 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 well fit by an ellipse with an area of 101 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis): icrs; ellipse(09h24m, -06d08m, 14.43d, 2.23d, 119.13d) Marginalized over the whole sky, the a posteriori luminosity distance estimate is 794 +/- 191 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] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and Ewing et al. (2023) arXiv:2305.05625 [3] Aubin et al. CQG 38, 095004 (2021) doi:10.1088/1361-6382/abe913 [4] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [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/39148. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...