TITLE: GCN CIRCULAR NUMBER: 25187 SUBJECT: LIGO/Virgo S190728q: Identification of a GW compact binary merger candidate DATE: 19/07/28 08:47:18 GMT FROM: Sarah Antier at APC The LIGO Scientific Collaboration and the Virgo Collaboration report: We identified the compact binary merger candidate S190728q during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2019-07-28 06:45:10.529 UTC (GPS time: 1248331528.529). The candidate was found by the GstLAL [1], CWB [2], MBTAOnline [3], and SPIIR [4] analysis pipelines. S190728q is an event of interest because its false alarm rate, as estimated by the online analysis, is 2.5e-23 Hz, or about one in 1e15 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S190728q The classification of the GW signal, in order of descending probability, is MassGap (52%), BBH (34%), NSBH (14%), Terrestrial (<1%), or BNS (<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 55 minutes after the candidate For the bayestar.fits.gz sky map, the 90% credible region is 543 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 795 +/- 197 Mpc (a posteriori mean +/- standard deviation). Note that both skymap and source-classification were updated for the event in favor of a triple detector event. 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] Klimenko et al. PRD 93, 042004 (2016)  [3] Adams et al. CQG 33, 175012 (2016)  [4] Qi Chu, PhD Thesis, The University of Western Australia (2017)  [5] Singer & Price PRD 93, 024013 (2016)