SUBJECT: LIGO/Virgo/KAGRA S5678: Updated Sky localization, Source Classification, EM Bright Classification, Coincidence with External Event, and Mass Estimate

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA
Collaboration along with the Fermi GBM Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) data around the time of the compact binary
merger (CBC) candidate S5678 (GCN Circular ***CITE ORIGINAL GCN ID, e.g.
25012***). Parameter estimation has been performed using Bilby [1] and a new
sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is
available for retrieval from the GraceDB event page:

https://gracedb.invalid/superevents/S5678

After parameter estimation by RapidPE-RIFT [2], the updated classification of
the GW signal, in order of descending probability, is BNS (78%), NSBH (22%),
BBH (<1%), or Terrestrial (<1%).

Based on posterior support from parameter estimation [1], under the assumption
that the candidate S5678 is astrophysical in origin, the probability that at
least one of the compact objects is consistent with a neutron star mass above
one solar mass (HasNS) is >99%. [3] Using the masses and spins inferred from
the signal, the probability of matter outside the final compact object
(HasRemnant) is <1%. [3] HasRemnant is assumed to be zero when the heavier
component mass is below 1 solar mass. 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 probability that the lighter compact
object is below 1 solar mass (HasSSM) is 50%.

The source chirp mass falls with highest probability in the bin (2.3, 3.0)
solar masses after parameter estimation [1], assuming the candidate is
astrophysical in origin.

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by
an ellipse with an area of 82 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(03h08m, -45d08m, 8.82d, 2.98d, 111.99d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate
is 522 +/- 102 Mpc (a posteriori mean +/- standard deviation).

A search performed by the RAVEN pipeline [4] found a temporal coincidence
between S5678 and a sub-threshold Fermi GBM candidate with ID 702818765 **CITE
ORIGINAL GCN FOR THE EXTERNAL CANDIDATE FROM https://gcn.nasa.gov/circulars,
e.g., (Bhalerao et al., GCN Circular XXXXX)**. The GRB candidate time is 2.1
seconds after the GW candidate event. The estimated joint false alarm rate for
the coincidence using just timing info before trials are applied is 3e-11 Hz,
or about one in 1e3 years. The GRB candidate was found during a joint targeted
search between the LIGO/Virgo/KAGRA collaboration and Fermi GBM, and has a
false alarm rate of 1.2e-06 Hz, or about one in 9 days.

Combined sky maps are also available:
 * combined-ext.multiorder.fits,0, an initial localization, distributed via GCN
and SCiMMA notices about 10 hours after the candidate event time.
 * combined-ext.multiorder.fits,1, an updated localization, distributed via GCN
and SCiMMA notices about 11 hours after the candidate event time.

For the combined-ext.multiorder.fits,1 sky map, the 90% credible region is 52
deg2. Considering the overlap of the individual sky maps, the estimated joint
false alarm rate for the spatial and temporal coincidence before trials are
applied is 4e-12 Hz, or about one in 1e4 years.

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] Ashton et al. ApJS 241, 27 (2019) doi:10.3847/1538-4365/ab06fc and
Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040
 [2] Rose et al. (2022) arXiv:2201.05263 and Pankow et al. PRD 92, 023002
(2015) doi:10.1103/PhysRevD.92.023002
 [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [4] Urban, A. L. 2016, Ph.D. Thesis https://dc.uwm.edu/etd/1218 and
Piotrzkowski, B. J. 2022, Ph.D. Thesis https://dc.uwm.edu/etd/3060