TITLE: GCN CIRCULAR NUMBER: 27963 SUBJECT: LIGO/Virgo MS200614bz: Test Identification of a binary neutron star candidate DATE: 20/06/16 12:09:06 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 MS200614bz during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2020-06-14 12:57:27.027 UTC (GPS time: 1276174665.027). The candidate was found by the GstLAL [1] analysis pipeline. MS200614bz is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.3e-08 Hz, or about one in 2 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/MS200614bz The classification of the GW signal, in order of descending probability, is BNS (62%), Terrestrial (38%), NSBH (0%), BBH (0%), or MassGap (0%). Assuming the candidate is astrophysical in origin, the probability that the lighter compact object has a mass < 3 solar masses (HasNS) is >99% [3]. Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is >99% [3]. 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 [2], * bayestar.fits.gz,1, an initial localization generated by BAYESTAR [2]. 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 4219 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 278 +/- 134 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] Singer & Price PRD 93, 024013 (2016) [3] Chatterjee et al. The Astrophysical Journal 896, 1 (2020)