TITLE: GCN CIRCULAR NUMBER: 34306 SUBJECT: LIGO/Virgo/KAGRA S230731an: Updated Sky Localization and EM Bright Classification, and Correction to Source Classification DATE: 23/08/01 01:24:15 GMT FROM: carl.haster@unlv.edu

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

We have conducted further analysis of the LIGO Hanford Observatory (H1) and LIGO Livingston Observatory (L1) data around the time of the compact binary merger (CBC) candidate S230731an (GCN Circular 34303). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S230731an

Based on posterior support from parameter estimation [1], under the assumption that the candidate S230731an is astrophysical in origin, the probability that the lighter compact object is consistent with a neutron star mass (HasNS) is <1%. [2] Using the masses and spins inferred from the signal, the probability of matter outside the final compact object (HasRemnant) is <1%. [2] Both HasNS and HasRemnant consider the support of several neutron star equations of state. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassgap) is <1%.

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 599 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 1001 +/- 242 Mpc (a posteriori mean +/- standard deviation).

The GCN Circular 34303 quoted incorrect values for the classification of the GW signal. These differ from the correct values found in the Initial Notice. The correct values from the Initial Notice are: BBH (81%), NSBH (18%), Terrestrial (<1%), or BNS (0%).

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/.

[1] Ashton et al. ApJS 241, 27 (2019) [2] Chatterjee et al. ApJ 896, 54 (2020)

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