# GCN Circular 34775

**Subject**

LIGO/Virgo/KAGRA S230927be: Identification of a GW compact binary merger candidate

**Date**

2023-09-27T16:18:47Z (a year ago)

**From**

brina.martinez@ligo.org

**Via**

Web form

```
The LIGO Scientific Collaboration, the Virgo Collaboration, and the
KAGRA Collaboration report:
We identified the compact binary merger candidate S230927be during
real-time processing of data from LIGO Hanford Observatory (H1) and
LIGO Livingston Observatory (L1) at 2023-09-27 15:38:32.919 UTC (GPS
time: 1379864330.919). The candidate was found by the CWB [1], GstLAL
[2], MBTA [3], PyCBC Live [4], and SPIIR [5] analysis pipelines.
S230927be 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:
https://gracedb.ligo.org/superevents/S230927be
The classification of the GW signal, in order of descending
probability, is BBH (>99%), Terrestrial (<1%), BNS (<1%), or NSBH
(<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%. [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 35 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 527
deg2. Marginalized over the whole sky, the a posteriori luminosity
distance estimate is 1056 +/- 308 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] 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)
```