TITLE: GCN CIRCULAR NUMBER: 18442 SUBJECT: LIGO/Virgo G194575: Identification of a GW CBC Candidate DATE: 15/10/22 20:03:45 GMT FROM: Leo Singer at NASA/GSFC Leo Singer (NASA/GSFC), Peter Shawhan (UMD), Nipuni Palliyaguru (TTU), B. S. Sathyaprakash (Cardiff), Alex Urban (UWM), Jeff Bartlett (LIGO Hanford), Travis Sadecki (LIGO Hanford), Mike Landry (LIGO Hanford), Joe Hanson (LIGO Livingston), Bryan Smith (LIGO Livingston), Brian O'Reilly (LIGO Livingston), Kipp Cannon (CITA), Gianluca Guidi (Urbino), Andy Lundgren (AEI/Hannover), Laura Nuttall (Syracuse), T. J. Massinger (Syracuse), Jessica McIver (Caltech), and Joshua Smith (Fullerton) report on behalf of the LIGO Scientific Collaboration and Virgo: The gstlal CBC analysis, which is sensitive to binary coalescence events from systems containing at least one neutron star, identified candidate G194575 during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2015-10-22 13:33:19.942 UTC (GPS time: 1129556016.942). Both the H1 and L1 detectors were in observation mode and operating normally. G194575 is an event of interest because its false alarm rate, as determined by the online analysis, is 9.65e-08 Hz or about one in 4 months, passing our stated alert threshold of ~1/month for CBC candidates. The event's properties can be found at this URL: https://gracedb.ligo.org/events/G194575 Fermi triggered (bn151022577) about 1 ks after the LIGO candidate, and Swift detected GRB 151022A (D’Avanzo et al., GCN 18436) about 2 ks after. Neither gamma-ray trigger’s localization is consistent with the rapid position reconstruction of the LIGO candidate. Based on the separation in time and sky position, we infer that these high-energy triggers are unrelated. One sky map is available at this time and can be retrieved from the GraceDB event page: skymap.fits.gz, an initial localization generated by BAYESTAR. The probability is concentrated in two main equatorial regions around right ascensions of 1h and 13h. About 30% of the probability is confined to the top 500 deg2. Updates on our analysis of this event will be sent as they become available.