TITLE: GCN CIRCULAR NUMBER: 41437 SUBJECT: LIGO/Virgo/KAGRA S250818k: Properties of the low-significance GW compact binary merger candidate potentially associated with AT 2025ulz DATE: 25/08/20 06:04:02 GMT FROM: gwangeon.seong@ligo.org

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration (LVK) report:

We identified the compact binary merger candidate S250818k during real-time processing of data from LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) at 2025-08-18 01:20:06.030 UTC (GPS time: 1439515224.030). The candidate was found by the GstLAL SSM [1] and PyCBC Live [2] analysis pipelines. Two LVK Preliminary GCN Notices were issued within 5 minutes after the candidate was identified.

Based on the analysis of gravitational-wave (GW) data alone, this candidate does not meet our criteria for a high-significance public alert as its false alarm rate is estimated by the online analysis to be 6.8e-08 Hz or about one in 5 months. However, we are issuing this Circular to confirm the properties of the GW candidate because of its potential association with the optical transient ZTF25abjmnps / AT 2025ulz (Stein et al, GCN 41414) which has been confirmed and reported to be fading and reddening rapidly (Busmann et al., GCN 41421; Hall et al., GCN 41433; Karambelkar et al., GCN 41436). The GW candidate’s properties can be found at this URL:

https://gracedb.ligo.org/superevents/S250818k

The classification of the GW candidate, in order of descending probability, is Terrestrial (71%), BNS (29%), NSBH (<1%), or BBH (<1%). Initial data quality checks found that there is no evidence for presence of a glitch or other data quality issues.

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%. [3] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is >99%. [3] 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 source chirp mass falls with highest probability in the bin (0.1, 0.87) solar masses, assuming the candidate is astrophysical in origin.

Three sky maps are available at this time and can be retrieved from the GraceDB event page: * bayestar.multiorder.fits,0, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 28 seconds after the candidate event time. * bayestar.multiorder.fits,1, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 5 minutes after the candidate event time. * bayestar.multiorder.fits,2, an initial localization generated by BAYESTAR [4], distributed via GCN and SCiMMA notices about 2 days after the candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,2. For the bayestar.multiorder.fits,2 sky map, the 90% credible region is 786 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 259 +/- 74 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] Hanna et al. (2024) arXiv 2412.10951 [2] Dal Canton et al. ApJ 923, 254 (2021) doi:10.3847/1538-4357/ac2f9a [3] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe [4] Singer & Price PRD 93, 024013 (2016) doi:10.1103/PhysRevD.93.024013

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