TITLE: GCN CIRCULAR NUMBER: 26906 SUBJECT: LIGO/Virgo S200128d: Identification of a GW compact binary merger candidate DATE: 20/01/28 02:56:16 GMT FROM: Erik Katsavounidis at MIT The LIGO Scientific Collaboration and the Virgo Collaboration report: We identified the compact binary merger candidate S200128d during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2020-01-28 02:20:11.903 UTC (GPS time: 1264213229.903). The candidate was found by the PyCBC Live [1], CWB [2], MBTAOnline [3], and GstLAL [4] analysis pipelines. S200128d is an event of interest because its false alarm rate, as estimated by the online analysis, is 1.6e-08 Hz, or about one in 1 year, 11 months. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S200128d 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%. Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. 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], distributed via GCN notice about 3 minutes after the candidate event time.  * bayestar.fits.gz,1, an initial localization generated by BAYESTAR [5], distributed via GCN notice about 9 minutes after the candidate event time. 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 2521 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 4031 +/- 1241 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] Nitz et al. PRD 98, 024050 (2018) [2] Klimenko et al. PRD 93, 042004 (2016) [3] Adams et al. CQG 33, 175012 (2016) [4] Messick et al. PRD 95, 042001 (2017) [5] Singer & Price PRD 93, 024013 (2016)