SUBJECT: LIGO/Virgo/KAGRA S5678: Updated Coincidence with External Event

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA
Collaboration along with the Fermi GBM Collaboration report:

We have conducted further analysis around the time of the compact binary merger
(CBC) candidate S5678 (GCN Circular ***CITE ORIGINAL GCN ID, e.g. 25012***).

A search performed by the RAVEN pipeline [1] found a temporal coincidence
between S5678 and a sub-threshold Fermi GBM candidate with ID 702818765 **CITE
ORIGINAL GCN FOR THE EXTERNAL CANDIDATE FROM https://gcn.nasa.gov/circulars,
e.g., (Bhalerao et al., GCN Circular XXXXX)**. The GRB candidate time is 2.1
seconds after the GW candidate event. The estimated joint false alarm rate for
the coincidence using just timing info before trials are applied is 3e-11 Hz,
or about one in 1e3 years. The GRB candidate was found during a joint targeted
search between the LIGO/Virgo/KAGRA collaboration and Fermi GBM, and has a
false alarm rate of 1.2e-06 Hz, or about one in 9 days.

Combined sky maps are also available:
 * combined-ext.multiorder.fits,0, an initial localization, distributed via GCN
and SCiMMA notices about 10 hours after the candidate event time.
 * combined-ext.multiorder.fits,1, an updated localization, distributed via GCN
and SCiMMA notices about 11 hours after the candidate event time.

For the combined-ext.multiorder.fits,1 sky map, the 90% credible region is 52
deg2. Considering the overlap of the individual sky maps, the estimated joint
false alarm rate for the spatial and temporal coincidence before trials are
applied is 4e-12 Hz, or about one in 1e4 years.

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] Urban, A. L. 2016, Ph.D. Thesis https://dc.uwm.edu/etd/1218 and
Piotrzkowski, B. J. 2022, Ph.D. Thesis https://dc.uwm.edu/etd/3060