SUBJECT: LIGO/Virgo/KAGRA S1235: Identification of a GW compact binary merger candidate possibly associated with CHIME candidate with ID 206742

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

We identified the compact binary merger candidate S1235 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.

S1235 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/S1235

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 (1.1, 1.2)
solar masses, assuming the candidate is astrophysical in origin.

Two GW-only 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 12 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).

A search performed by the RAVEN pipeline [10] found a temporal coincidence
between S1235 and a CHIME candidate with ID 206742 **CITE ORIGINAL GCN FOR THE
EXTERNAL CANDIDATE FROM https://gcn.nasa.gov/circulars, e.g., (Bhalerao et al.,
GCN Circular XXXXX)**. The fast radio burst candidate time is 4.6 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. RAVEN has also identified additional detections from
Einstein Probe, Fermi GBM, and Swift/BAT.

A combined sky map is also available:
 * combined-ext.multiorder.fits, an initial localization, distributed via GCN
and SCiMMA notices about 12 minutes after the candidate event time.

For the combined-ext.multiorder.fits sky map, the 90% credible region is <1
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 1.8e-11 Hz, or about one in 1e3 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] 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
 [10] 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