SUBJECT: LIGO/Virgo/KAGRA S1234: 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 S1234 during real-time
processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston
Observatory (L1) at 2018-06-28 03:08:04.741 UTC (GPS time: 1214190502.741). The
candidate was found by the Aframe [1], GstLAL [2], MBTA [3], MLy [4], and PyCBC
Live [5] analysis pipelines. An early-warning alert was issued for this
candidate, detected by the GstLAL [6], MBTA [3], and PyCBC Live [7] early-
warning pipelines.

S1234 is an event of interest because its false alarm rate, as estimated by the
online analysis, is 9.1e-14 Hz, or about one in 1e6 years. The event's
properties can be found at this URL:

https://gracedb.invalid/superevents/S1234

The classification of the GW signal, in order of descending probability, is BNS
(95%), Terrestrial (4%), NSBH (1%), or BBH (<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 >99%. [8] Using the masses and spins inferred from the signal, the
probability of matter outside the final compact object (HasRemnant) is >99%.
[8] 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 20%.

The source chirp mass falls with highest probability in the bin (3.0, 5.5)
solar masses, assuming the candidate is astrophysical in origin.

Two sky maps are available at this time and can be retrieved from the GraceDB
event page:
 * bayestar.multiorder.fits,0, an early-warning localization generated by
BAYESTAR [9], distributed via GCN and SCiMMA notices about a minute before the
candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR
[9], distributed via GCN and SCiMMA notices about 11 minutes after the
candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For the
bayestar.multiorder.fits,1 sky map, the 90% credible region is 24218 deg2.
Marginalized over the whole sky, the a posteriori luminosity distance estimate
is 29 +/- 9 Mpc (a posteriori mean +/- standard deviation).

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] Marx et al. PRD 111, 042010 (2025) doi:10.1103/PhysRevD.111.042010
 [2] Tsukada et al. PRD 108, 043004 (2023) doi:10.1103/PhysRevD.108.043004 and
Ewing et al. PRD 109, 042008 (2024) doi:10.1103/PhysRevD.109.042008
 [3] Alléné et al. CQG 42, 105009 (2025) doi:10.1088/1361-6382/add234
 [4] Skliris et al. PRD 110, 104034 (2024) doi:10.1103/PhysRevD.110.104034
 [5] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a
 [6] Sachdev et al. ApJL 905, 2 (2020) doi:10.3847/2041-8213/abc753
 [7] Nitz A. H., Schäfer M., Dal Canton T. ApJL 902, 2 (2020)
doi:10.3847/2041-8213/abbc10
 [8] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe
 [9] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013