TITLE: GCN CIRCULAR NUMBER: 39605 SUBJECT: LIGO/Virgo/KAGRA S250207bg: Updated Sky localization DATE: 25/03/06 16:37:39 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 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 S250207bg (GCN Circulars 39201 and 39242). 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/S250207bg
For the Bilby.offline1.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 19 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(10h51m, +36d04m, 3.72d, 1.66d, 139.66d) Marginalized over the whole sky, the a posteriori luminosity distance estimate is 180 +/- 38 Mpc (a posteriori mean +/- standard deviation).
At the time of the candidate, the Hanford detector was being brought online and was not in observing mode. However, it was determined that the Hanford data are sufficiently robust to inform our estimate of the sky localization. The well-measured arrival time of the signal at Hanford, while consistent with the Livingston and Virgo data, breaks the degeneracy in sky localization inherent in a 2-detector analysis, and significantly shifts the posterior distribution with respect to the earlier localization. Further investigations are ongoing to understand how the operational state of Hanford at the time of the event impacts this analysis. The estimated sky localization may change slightly based on these studies, but this skymap represents our best understanding of the event at this time.
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. (2023) arXiv:2307.13380
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