TITLE: GCN CIRCULAR NUMBER: 26334 SUBJECT: LIGO/Virgo S191204r: Identification of a GW compact binary merger candidate DATE: 19/12/04 18:41:47 GMT FROM: Deep Chatterjee at University of Wisconsin, Milwaukee The LIGO Scientific Collaboration and the Virgo Collaboration report: We identified the compact binary merger candidate S191204r during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2019-12-04 17:15:26.092 UTC (GPS time: 1259514944.092). The candidate was found by the GstLAL [1], CWB [2], MBTAOnline [3], and SPIIR [4] analysis pipelines. S191204r is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.1e-25 Hz, or about one in 1e17 years. The event's properties can be found at this URL: https://gracedb.ligo.org/superevents/S191204r At the time of the event the Virgo detector was performing calibration injections, and was in low noise mode but not observing. At the exact time of the event Virgo was between calibration injections and the data was undisturbed. After discussion we agreed that the data was usable for sky localization. The classification of the GW signal, in order of descending probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), MassGap (<1%), or NSBH (<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%. Three 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 42 minutes after the candidate event time. * bayestar.fits.gz,1, an initial localization generated by BAYESTAR [5], distributed via GCN notice about 54 minutes after the candidate event time. * bayestar.fits.gz,2, an initial localization generated by BAYESTAR [5], distributed via GCN notice about an hour after the candidate event time. The preferred sky map at this time is bayestar.fits.gz,2. For the bayestar.fits.gz,2 sky map, the 90% credible region is 103 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 678 +/- 149 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] 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) [GCN OPS NOTE (04dec19): The typo extra "SUBJECT:" was removed from the SUBJECT:-line.]