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GCN Circular 41033

Subject
LIGO/Virgo/KAGRA S250704ab: Updated Sky localization and EM Bright Classification
Event
LIGO/Virgo/KAGRA S250704ab
Date
2025-07-08T15:59:56Z (4 days ago)
From
lucy.thomas@ligo.org
Via
Web form
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 S250704ab (GCN Circular 40935Loading...
 
 ). 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/S250704ab

Based on posterior support from parameter estimation [1], under the assumption that the candidate S250704ab 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 for maximum neutron star mass. The probability that either of the binary components lies between 3 and 5 solar masses (HasMassGap) is <1%. The probability that the lighter compact object is below 1 solar mass (HasSSM) is <1%.

The source chirp mass falls with highest probability in the bin (5.5, 11.0) solar masses after parameter estimation [1], assuming the candidate is astrophysical in origin.

For the Bilby.multiorder.fits,0 sky map, the 90% credible region is well fit by an ellipse with an area of 57 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(05h40m, +44d43m, 7.74d, 2.35d, 77.56d)
Marginalized over the whole sky, the a posteriori luminosity distance estimate is 531 +/- 101 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/ab06fcLoading...
 
  and Morisaki et al. PRD 108, 123040 (2023) doi:10.1103/PhysRevD.108.123040Loading...
 
 
 [2] Chatterjee et al. ApJ 896, 54 (2020) doi:10.3847/1538-4357/ab8dbeLoading...