TITLE: GCN CIRCULAR NUMBER: 24069 SUBJECT: LIGO/Virgo S190408an: Identification of a GW binary merger candidate DATE: 19/04/08 21:36:11 GMT FROM: Leo Singer at GSFC The LIGO Scientific Collaboration and the Virgo Collaboration report: We identified the compact binary merger candidate S190408an during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2019-04-08 18:18:02.288 UTC (GPS time: 1238782700.288). The candidate was found by the GstLAL [1], SPIIR [2], CWB [3], MBTAOnline [4], and PyCBC Live [5] analysis pipelines. S190408an is a candidate of interest because its false alarm rate, as determined by the online analysis, is less than one in 100 years. The candidate's properties can be found at this URL: https://gracedb.ligo.org/superevents/S190408an The classification of the signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), 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 evidence against matter outside the final compact object(HasRemnant: 12%). We believe that the latter quantity (HasRemnant) may be overestimated; we are reviewing it and will provide an update when available. One skymap is available at this time and can be retrieved from the GraceDB candidate page: * bayestar.fits.gz, a preliminary localization generated by BAYESTAR [6], distributed via GCN notice about 34 minutes after the candidate. For the bayestar.fits.gz skymap, the 90% credible region is 387 deg2. Marginalized over the whole sky, the luminosity distance estimate is 1473 +/- 358 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] Hooper et al. PRD 86, 024012 (2012) [3] Klimenko et al. PRD 93, 042004 (2016) [4] Adams et al. CQG 33, 175012 (2016) [5] Nitz et al. PRD 98, 024050 (2018) [6] Singer & Price PRD 93, 024013 (2016)