SUBJECT: LIGO/Virgo/KAGRA S2468: Identification of a GW unmodeled transient candidate possibly associated with SNEWS candidate with ID 1000215

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA
Collaboration report:

We identified the unmodeled transient candidate S2468 during real-time
processing of data from LIGO Hanford Observatory (H1), LIGO Livingston
Observatory (L1), and Virgo Observatory (V1) at 2018-06-28 03:08:03.740 UTC
(GPS time: 1214190501.740). The candidate was found by the cWB [1] and oLIB [2]
analysis pipelines.

S2468 is an event of interest because its false alarm rate, as estimated by the
online analysis, is 7.8e-08 Hz, or about one in 4 months. The event's
properties can be found at this URL:

https://gracedb.invalid/superevents/S2468

One sky map is available at this time and can be retrieved from the GraceDB
event page:
 * cwb.multiorder.fits, an initial localization generated by cWB [1],
distributed via GCN and SCiMMA notices about 4 years after the candidate event
time.

For the cwb.multiorder.fits 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(10h47m, +31d03m, 10.59d, 1.71d, 84.71d)

A search performed by the RAVEN pipeline [3] found a temporal coincidence
between S2468 and a SNEWS supernova candidate with ID 1000215 **CITE ORIGINAL
GCN FOR THE EXTERNAL CANDIDATE FROM https://gcn.nasa.gov/circulars, e.g.,
(Bhalerao et al., GCN Circular XXXXX)**. The neutrino candidate time is 2.0
seconds after the GW candidate event.

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] Klimenko et al. PRD 93, 042004 (2016) doi:10.1103/PhysRevD.93.042004
 [2] Lynch et al. PRD 95, 104046 (2017) doi:10.1103/PhysRevD.95.104046
 [3] 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