TITLE: GCN CIRCULAR NUMBER: 27965 SUBJECT: LIGO/Virgo MS200615ce: Identification of a test binary black hole candidate DATE: 20/06/16 13:07:05 GMT FROM: Surabhi Sachdev at LVC The LIGO Scientific Collaboration and the Virgo Collaboration report: *** This is a test of the Early Warning alert system resulting from archival O3 data. Times and sky localizations are fictitious. *** We identified the compact binary merger candidate MS200615ce during real-time processing of data from LIGO Hanford Observatory (H1) and Virgo Observatory (V1) at 2020-06-15 14:33:36.473 UTC (GPS time: 1276266834.473). The candidate was found by the GstLAL [1], PyCBC Live [2], and MBTAOnline [3] analysis pipelines. MS200615ce is an event of interest because its false alarm rate, as estimated by the online analysis, is 9e-19 Hz, or about one in 1e11 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/MS200615ce The classification of the GW signal, in order of descending probability, is MassGap (>99%), Terrestrial (<1%), BNS (<1%), BBH (<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 19%. 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 [4], distributed via GCN notice about 52 seconds after the candidate event time. * bayestar.fits.gz,1, an initial localization generated by BAYESTAR [4], distributed via GCN notice about 5 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 354 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 330 +/- 79 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] Nitz et al. PRD 98, 024050 (2018) [3] Adams et al. CQG 33, 175012 (2016) [4] Singer & Price PRD 93, 024013 (2016) [5] Singer & Price PRD 93, 024013 (2016) [6] Chatterjee et al. The Astrophysical Journal 896, 1 (2020)