TITLE: GCN CIRCULAR NUMBER: 41440 SUBJECT: LIGO/Virgo/KAGRA S250818k: Updated Sky localization and EM Bright Classification DATE: 25/08/20 10:12:16 GMT FROM: Soichiro Morisaki at U. of Tokyo soichiro.morisaki@ligo.org

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

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S250818k (GCN Circular 41437). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline0.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

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

Based on posterior support from parameter estimation [1], under the assumption that the candidate S250818k 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 80%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is 80%. [2] 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 7%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is >99%.

The source chirp mass falls with highest probability in the bin (0.1, 0.87) solar masses after parameter estimation [1], assuming the candidate is astrophysical in origin.

For the Bilby.offline0.multiorder.fits,0 sky map, the 90% credible region is 949 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 237 +/- 62 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] 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 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbe

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