SUBJECT: LIGO/Virgo/KAGRA S2469: Identification of a GW compact binary merger candidate

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA
Collaboration report:

We identified the compact binary merger candidate S2469 during real-time
processing of data from LIGO Hanford Observatory (H1), LIGO Livingston
Observatory (L1), and Virgo Observatory (V1) at 2018-06-28 03:08:03.740 UTC
(GPS time: 1214190501.740). The candidate was found by the cWB BBH [1] and
PyCBC Live [2] analysis pipelines.

S2469 is an event of interest because its false alarm rate, as estimated by the
online analysis, is 7.8e-08 Hz, or about one in 4 months. The event's
properties can be found at this URL:

https://gracedb.invalid/superevents/S2469

The classification of the GW signal, in order of descending probability, is BBH
(>99%), BNS (<1%), NSBH (<1%), or Terrestrial (<1%).

Assuming the candidate is astrophysical in origin, the probability that at
least one of the compact objects is consistent with a neutron star mass (HasNS)
is <1%. [3] Using the masses and spins inferred from the signal, the
probability of matter outside the final compact object (HasRemnant) is <1%. [3]
Both HasNS and HasRemnant consider the support of several neutron star
equations of state for maximum neutron star mass. The probability that either
of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%.

The redshifted (detector frame) chirp mass falls with highest probability in
the bin (3.0, 5.5) solar masses, assuming the candidate is astrophysical in
origin. For cWB BBH events, no distance or redshift information is available in
low latency.

One sky map is available at this time and can be retrieved from the GraceDB
event page:
 * cwb.multiorder.fits, an initial localization generated by cWB [4],
distributed via GCN and SCiMMA notices about 4 years after the candidate event
time.

For the cwb.multiorder.fits sky map, the 90% credible region is well fit by an
ellipse with an area of 57 deg2 described by the following DS9 region (right
ascension, declination, semi-major axis, semi-minor axis, position angle of the
semi-minor axis):
   icrs; ellipse(10h47m, +31d03m, 10.59d, 1.71d, 84.71d)

For further information about analysis methodology and the contents of this
alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide
https://emfollow.docs.ligo.org/.

 [1] T. Mishra et al. PRD 105, 083018 (2022) doi:10.1103/PhysRevD.105.083018
 [2] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [4] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004