GCN Circular 27965

The LIGO Scientific Collaboration and the Virgo Collaboration report:

*** This is a test of the Early Warning alert system resulting from archival
O3 data. Times and sky localizations are fictitious. ***

We identified the compact binary merger candidate MS200615ce during
real-time processing of data from LIGO Hanford Observatory (H1) and
Virgo Observatory (V1) at 2020-06-15 14:33:36.473 UTC (GPS time:
1276266834.473). The candidate was found by the GstLAL [1], PyCBC Live
[2], and MBTAOnline [3] analysis pipelines.

MS200615ce is an event of interest because its false alarm rate, as
estimated by the online analysis, is 9e-19 Hz, or about one in 1e11
years. The event's properties can be found at this URL:
https://gracedb.ligo.org/superevents/MS200615ce

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

Assuming the candidate is astrophysical in origin, the probability
that the lighter compact object has a mass < 3 solar masses (HasNS) is
19%. Using the masses and spins inferred from the signal, the
probability of matter outside the final compact object (HasRemnant) is
<1%.

Two sky maps are available at this time and can be retrieved from the
GraceDB event page:
 * bayestar.fits.gz,0, an initial localization generated by BAYESTAR [4],
distributed via GCN notice about 52 seconds after the candidate event time.
 * bayestar.fits.gz,1, an initial localization generated by BAYESTAR [4],
distributed via GCN notice about 5 minutes after the candidate event time.

The preferred sky map at this time is bayestar.fits.gz,1. For the
bayestar.fits.gz,1 sky map, the 90% credible region is 354 deg2.
Marginalized over the whole sky, the a posteriori luminosity distance
estimate is 330 +/- 79 Mpc (a posteriori mean +/- standard deviation).

For further information about analysis methodology and the contents of
this alert, refer to the LIGO/Virgo Public Alerts User Guide
<https://emfollow.docs.ligo.org/userguide/>.

 [1] Messick et al. PRD 95, 042001 (2017)
 [2] Nitz et al. PRD 98, 024050 (2018)
 [3] Adams et al. CQG 33, 175012 (2016)
 [4] Singer & Price PRD 93, 024013 (2016)
 [5] Singer & Price PRD 93, 024013 (2016)
 [6] Chatterjee et al. The Astrophysical Journal 896, 1 (2020)