TITLE: GCN CIRCULAR NUMBER: 24621 SUBJECT: LIGO/Virgo S190521g: Identification of a GW compact binary merger candidate DATE: 19/05/21 04:19:51 GMT FROM: Geoffrey Mo at LIGO The LIGO Scientific Collaboration and the Virgo Collaboration report: We identified the compact binary merger candidate S190521g during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2019-05-21 03:02:29.447 UTC (GPS time: 1242442967.447). The candidate was found by the PyCBC Live [1], CWB [2], GstLAL [3], and SPIIR [4] analysis pipelines. S190521g is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.8e-09 Hz, or about one in 8 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S190521g 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, there is strong evidence against the lighter compact object having a mass < 3 solar masses (HasNS: <1%). Using the masses and spins inferred from the signal, there is strong evidence against matter outside the final compact object (HasRemnant: <1%). One sky map is available at this time and can be retrieved from the GraceDB event page: * bayestar.fits.gz, an updated localization generated by BAYESTAR [5], distributed via GCN notice about 6 minutes after the candidate For the bayestar.fits.gz sky map, the 90% credible region is 1163 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 663 +/- 156 Mpc (a posteriori mean +/- standard deviation). We have reason to believe that the distance may be underestimated because different search pipelines have reported a range of distances. The two-dimensional sky map (right ascension and declination) is in good agreement across pipelines. Offline analyses to resolve this issue are ongoing. For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo Public Alerts User Guide . [1] Nitz et al. PRD 98, 024050 (2018) [2] Klimenko et al. PRD 93, 042004 (2016) [3] Messick et al. PRD 95, 042001 (2017) [4] Qi Chu, PhD Thesis, The University of Western Australia (2017) [5] Singer & Price PRD 93, 024013 (2016)