# GCN Circular 34546

**Subject**

LIGO/Virgo/KAGRA S230825k: Identification of a GW compact binary merger candidate

**Date**

2023-08-25T06:42:29Z (6 months ago)

**From**

Hajime Sotani at RIKEN <h.sotani@gmail.com>

**Via**

Web form

```
The LIGO Scientific Collaboration, the Virgo Collaboration, and the
KAGRA Collaboration report:
We identified the compact binary merger candidate S230825k during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2023-08-25 04:13:34.546 UTC (GPS
time: 1376972032.546). The candidate was found by the CWB [1], GstLAL
[2], and MBTA [3] analysis pipelines.
S230825k is an event of interest because its false alarm rate, as
estimated by the online analysis, is 2.4e-09 Hz, or about one in 13
years. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/S230825k
The classification of the GW signal, in order of descending
probability, is BBH (>99%), Terrestrial (<1%), NSBH (<1%), or BNS
(<1%).
Assuming the candidate is astrophysical in origin, the probability
that the lighter compact object is consistent with a neutron star mass
(HasNS) is <1%. [4] Using the masses and spins inferred from the
signal, the probability of matter outside the final compact object
(HasRemnant) is <1%. [4] 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 6%.
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 [5], distributed via GCN notice about 27 seconds after the
candidate event time.
* bayestar.multiorder.fits,1, an initial localization generated by
BAYESTAR [5], 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
2936 deg2. Marginalized over the whole sky, the a posteriori
luminosity distance estimate is 7075 +/- 2287 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/userguide/.
[1] Klimenko et al. PRD 93, 042004 (2016)
[2] Tsukada et al. arXiv:2305.06286 (2023) and Ewing et al.
arXiv:2305.05625 (2023)
[3] Aubin et al. CQG 38, 095004 (2021)
[4] Chatterjee et al. ApJ 896, 54 (2020)
[5] Singer & Price PRD 93, 024013 (2016)
```