The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:
We identified the compact binary merger candidate S230518h during real-time processing of data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) at 2023-05-18 12:59:08.167 UTC (GPS time: 1368449966.167). The candidate was found by the PyCBC Live , GstLAL , and MBTAOnline  analysis pipelines.
The LIGO detectors are currently operating in an "engineering run" mode prior to the start of the O4 observing run. The data being collected at the time of this candidate is believed to be of good quality based on preliminary checks, but requires further investigation. A decision was made to alert the community promptly, with this caveat, due to the potential significance of this candidate.
S230518h is an event of interest because its false alarm rate, as estimated by the online analysis, is 3.2e-10 Hz, or about one in 98 years. The event's properties can be found at this URL:
The classification of the GW signal, in order of descending probability, is NSBH (86%), Terrestrial (10%), BBH (4%), or BNS (<1%).
Assuming the candidate is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is >99%.  Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is < 1%. Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that any one of the binary components lie between 3 to 5 solar mass (HasMassgap) is < 1%.
One sky map is available at this time and can be retrieved from the GraceDB event page:
* bayestar.multiorder.fits, an initial localization generated by BAYESTAR , distributed via GCN Notice about 39 minutes after the candidate event time.
For the bayestar.multiorder.fits sky map, the 90% credible region is 1002 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 276 +/- 79 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/userguide/>.
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