TITLE: GCN CIRCULAR NUMBER: 42375 SUBJECT: LIGO/Virgo/KAGRA S251021u: Updated Sky localization DATE: 25/10/21 16:18:24 GMT FROM: mattia.emma.2022@live.rhul.ac.uk

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 S251021u (GCN Circular 42373). Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.multiorder.fits,0, distributed via GCN and SCiMMA notices, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S251021u

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is 3859 deg2. Marginalized over the whole sky, the a posteriori luminosity distance estimate is 5884 +/- 2089 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

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