Skip to main content
New Announcement Feature, Code of Conduct, Circular Revisions. See news and announcements

GCN Circular 34113

LIGO/Virgo/KAGRA S230628ax: Identification of a GW compact binary merger candidate
2023-06-29T00:59:04Z (a year ago)
David Hui at Chungnam National University <>
The LIGO Scientific Collaboration, the Virgo Collaboration, and the
KAGRA Collaboration report:

We identified the compact binary merger candidate S230628ax during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2023-06-28 23:12:00.373 UTC (GPS
time: 1372029138.373). The candidate was found by the CWB [1], MBTA
[2], GstLAL [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.

S230628ax is an event of interest because its false alarm rate, as
estimated by the online analysis, is 3.2e-10 Hz, or about one in 1e2
years. The event's properties can be found at this URL:

The classification of the GW signal, in order of descending
probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH

Assuming the candidate is astrophysical in origin, the probability
that the lighter compact object is consistent with a neutron star mass
(HasNS) is <1%. [6] Using the masses and spins inferred from the
signal, the probability of matter outside the final compact object
(HasRemnant) is <1%. [6] 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%.

Two sky maps are available at this time and can be retrieved from the
GraceDB event page:
 * bayestar.multiorder.fits,0, an initial localization generated by
BAYESTAR [7], distributed via GCN notice about 26 seconds after the
candidate event time.
 * bayestar.multiorder.fits,1, an initial localization generated by
BAYESTAR [7], distributed via GCN notice about 5 minutes after the
candidate event time.

The preferred sky map at this time is bayestar.multiorder.fits,1. For
the bayestar.multiorder.fits,1 sky map, the 90% credible region is 705
deg2. Marginalized over the whole sky, the a posteriori luminosity
distance estimate is 2047 +/- 585 Mpc (a posteriori mean +/- standard

For further information about analysis methodology and the contents of
this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide

 [1] Klimenko et al. PRD 93, 042004 (2016)
 [2] Aubin et al. CQG 38, 095004 (2021)
 [3] Tsukada et al. arXiv:2305.06286 (2023) and Ewing et al.
arXiv:2305.05625 (2023)
 [4] Dal Canton et al. ApJ 923, 254 (2021)
 [5] Chu et al. PRD 105, 024023 (2022)
 [6] Chatterjee et al. ApJ 896, 54 (2020)
 [7] Singer & Price PRD 93, 024013 (2016)

Looking for U.S. government information and services? Visit