GCN Circular 25753

The LIGO Scientific Collaboration and the Virgo Collaboration report:

We identified the compact binary merger candidate S190915ak during
real-time processing of data from LIGO Hanford Observatory (H1), LIGO
Livingston Observatory (L1), and Virgo Observatory (V1) at 2019-09-15
23:57:02.691 UTC (GPS time: 1252627040.691). The candidate was found
by the GstLAL [1], SPIIR [2], CWB [3], and MBTAOnline [4] analysis
pipelines.

S190915ak is an event of interest because its false alarm rate, as
estimated by the online analysis, is 9.7e-10 Hz, or about one in 32
years. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S190915ak

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, 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 trigger time.
For the bayestar.fits.gz sky map, the 90% credible region is 528 deg2.
Marginalized over the whole sky, the a posteriori luminosity distance
estimate is 1557 +/- 381 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
<https://emfollow.docs.ligo.org/userguide/>.

 [1] Messick et al. PRD 95, 042001 (2017)
 [2] Qi Chu, PhD Thesis, The University of Western Australia (2017)
 [3] Klimenko et al. PRD 93, 042004 (2016)
 [4] Adams et al. CQG 33, 175012 (2016)
 [5] Singer & Price PRD 93, 024013 (2016)

[GCN OPS NOTE(15sep19): Per author's request, "trigger time." was added
to the end of the first sentence in the fifth paragraph.]