TITLE:   GCN CIRCULAR
NUMBER:  37687
SUBJECT: LIGO/Virgo/KAGRA S240925n: Updated Sky localization
DATE:    24/10/03 03:33:37 GMT
FROM:    Sylvia Biscoveanu at Northwestern CIERA <sylvia.biscoveanu@ligo.org>

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

We have resolved the problem with the calibration of the LIGO Hanford instrument around the time of the compact binary merger (CBC) candidate S240925n (GCN Circulars 37604 and 37607). We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of this candidate. Parameter estimation has been performed using Bilby [1] and a new sky map, Bilby.offline1.multiorder.fits,0, distributed via GCN Notice, is available for retrieval from the GraceDB event page:

https://gracedb.ligo.org/superevents/S240925n

For the Bilby.offline1.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 17 deg2 described by the following DS9 region (right ascension, declination, semi-major axis, semi-minor axis, position angle of the semi-minor axis):  
   icrs; ellipse(19h26m, +06d35m, 5.61d, 0.99d, 124.67d)  
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 331 +/- 70 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