TITLE: GCN CIRCULAR NUMBER: 27977 SUBJECT: LIGO/Virgo MS200614ct: Identification of a test binary black hole candidate DATE: 20/06/18 04:11:20 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 MS200614ct during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2020-06-14 15:30:50.363 UTC (GPS time: 1276183868.363). The candidate was found by the MBTAOnline [1], CWB [2], PyCBC Live [3], and GstLAL [4] analysis pipelines. MS200614ct is an event of interest because its false alarm rate, as estimated by the online analysis, is 9.1e-09 Hz, or about one in 3 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/MS200614ct The classification of the GW signal, in order of descending probability, is BBH (97%), Terrestrial (3%), BNS (<1%), NSBH (<1%), or MassGap (<1%). Assuming the candidate is astrophysical in origin, the probability that the lighter compact object has a mass < 3 solar masses (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]. 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 [5], * bayestar.fits.gz,1, an initial localization generated by BAYESTAR [5]. 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 841 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1998 +/- 526 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] Adams et al. CQG 33, 175012 (2016) [2] Klimenko et al. PRD 93, 042004 (2016) [3] Nitz et al. PRD 98, 024050 (2018) [4] Messick et al. PRD 95, 042001 (2017) [5] Singer & Price PRD 93, 024013 (2016) [6] Chatterjee et al. The Astrophysical Journal 896, 1 (2020)