TITLE: GCN CIRCULAR NUMBER: 27358 SUBJECT: LIGO/Virgo S200311bg: Identification of a GW compact binary merger candidate DATE: 20/03/11 12:28:04 GMT FROM: Geoffrey Mo at MIT The LIGO Scientific Collaboration and the Virgo Collaboration report: We identified the compact binary merger candidate S200311bg during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2020-03-11 11:58:53.398 UTC (GPS time: 1267963151.398). The candidate was found by the GstLAL [1], CWB [2], PyCBC Live [3], SPIIR [4], and MBTAOnline [5] analysis pipelines. S200311bg is an event of interest because its false alarm rate, as estimated by the online analysis, is 8.9e-26 Hz, or about one in 1e18 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S200311bg The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), MassGap (<1%), or NSBH (<1%). Assuming the candidate is astrophysical in origin, the probability that the lighter compact object has a mass < 3 solar masses (HasNS) is <1%. Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. Two sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.fits.gz,0, an initial localization generated by BAYESTAR [6], distributed via GCN notice about 2 minutes after the candidate event time. * bayestar.fits.gz,1, an initial localization generated by BAYESTAR [6], distributed via GCN notice about 10 minutes after the candidate event time. The preferred sky map at this time is bayestar.fits.gz,1. For the bayestar.fits.gz,1 sky map, the 90% credible region is well fit by an ellipse with an area of 52 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(00h08m, -07d27m, 8.54d, 1.98d, 72.86d) Marginalized over the whole sky, the a posteriori luminosity distance estimate is 924 +/- 188 Mpc (a posteriori mean +/- standard deviation). For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo Public Alerts User Guide . [1] Messick et al. PRD 95, 042001 (2017) [2] Klimenko et al. PRD 93, 042004 (2016) [3] Nitz et al. PRD 98, 024050 (2018) [4] Qi Chu, PhD Thesis, The University of Western Australia (2017) [5] Adams et al. CQG 33, 175012 (2016) [6] Singer & Price PRD 93, 024013 (2016)