TITLE: GCN CIRCULAR NUMBER: 41364 SUBJECT: LIGO/Virgo/KAGRA S250814bg: Identification of a GW compact binary merger candidate possibly associated with GRB 250814A detected by Fermi GBM and Swift BAT DATE: 25/08/14 10:31:50 GMT FROM: Sreeta Roy at University of Warsaw <sreeta.roy@ligo.org> The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration with the Fermi GBM team and Swift team report: We identified the compact binary merger candidate S250814bg during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-14 07:35:05.587 UTC (GPS time: 1439192123.587). The candidate was found by the GstLAL [1] analysis pipeline. Based on the analysis of gravitational-wave data alone, this candidate does not meet our criteria for a high-significance public alert as its false alarm rate is estimated by the online analysis to be 0.00012 Hz or about one in 2 hours. However, a search performed by the RAVEN pipeline found a significant coincidence between this candidate and Fermi GBM trigger with ID 776849714, which is associated with GRB 250814A. The event’s properties can be found at this URL: https://gracedb.ligo.org/superevents/S250814bg The classification of the GW signal, in order of descending probability, is Terrestrial (>99%), BBH (<1%), NSBH (<1%), or BNS (<1%). Excess noise was present in the L1 detector at the time of this candidate, which may affect the parameters or the significance of the candidate. Assuming the candidate is astrophysical in origin, the probability that at least one of the compact objects 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 for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is 3%. The source chirp mass falls with highest probability in the bin (22.0, 44.0) solar masses, assuming the candidate is astrophysical in origin. Two 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 and SCiMMA notices about 2 minutes after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [3], distributed via GCN and SCiMMA notices about 6 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 7867 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 13504 +/- 6537 Mpc (a posteriori mean +/- standard deviation). A search performed by the RAVEN pipeline [4] found a temporal coincidence between S250814bg and a Fermi GBM trigger with ID 776849714, which is associated with GRB 250814A (Fermi GBM Team, GCN Circular 41357). The GRB trigger time is 3.7 seconds after the GW candidate event. The estimated joint false alarm rate for the coincidence using just timing info before trials are applied is 7.5e-09 Hz, or about one in 4 years. Combined sky maps are also available: * combined-ext.multiorder.fits,0, an initial localization, distributed via GCN and SCiMMA notices about 2 minutes after the candidate event time. * combined-ext.multiorder.fits,1, an initial localization, distributed via GCN and SCiMMA notices about 6 minutes after the candidate event time. * combined-ext.multiorder.fits,2, an initial localization, distributed via GCN and SCiMMA notices about an hour after the candidate event time. For the combined-ext.multiorder.fits,2 sky map, the 90% credible region is 471 deg2. Considering the overlap of the individual sky maps, the estimated joint false alarm rate for the spatial and temporal coincidence before trials are applied is 1.5e-08 Hz, or about one in 2 years. After including the trials factor of 30, the joint false alarm rate becomes 4.5e-07 Hz, or about 14 per year. Moreover, the GW candidate is 6 seconds after the Swift-BAT trigger 1342439, associated with the same GRB 250814A (Swift Team, GCN Circular 41358). The Swift-BAT GRB is spatio-temporal coincident with the Fermi GBM one, therefore they can be reliably considered the same astrophysical event. Considering the Swift-BAT localization, the estimated joint false alarm rate between the Swift-BAT and the GW candidate for the spatial and temporal coincidence before trials are applied is 1.0e-08 Hz, or about one in 3 years. After including the trials factor of 30, the joint false alarm rate becomes 3.0e-07 Hz, or about 9 per year. 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. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008 [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] 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/41364. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...