SUBJECT: LIGO/Virgo/KAGRA S5678: Updated Source Classification, EM Bright Classification, and Mass Estimate

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA
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***).

After parameter estimation by RapidPE-RIFT [1], 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 [2], 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 [2], assuming the candidate is
astrophysical in origin.

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] Rose et al. (2022) arXiv:2201.05263 and Pankow et al. PRD 92, 023002
(2015) doi:10.1103/PhysRevD.92.023002
 [2] 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
 [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe