GCN Circular 27963

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 MS200614bz during real-time
processing of data from LIGO Hanford Observatory (H1), LIGO Livingston
Observatory (L1), and Virgo Observatory (V1) at 2020-06-14 12:57:27.027 UTC
(GPS time: 1276174665.027). The candidate was found by the GstLAL [1]
analysis pipeline.

MS200614bz is an event of interest because its false alarm rate, as estimated
by the online analysis, is 1.3e-08 Hz, or about one in 2 years. The event's
properties can be found at this URL:
https://gracedb.ligo.org/superevents/MS200614bz

The classification of the GW signal, in order of descending probability, is BNS
(62%), Terrestrial (38%), NSBH (0%), BBH (0%), or MassGap (0%).

Assuming the candidate is astrophysical in origin, the probability that the
lighter compact object has a mass < 3 solar masses (HasNS) is
>99%  [3]. Using the masses and spins inferred from the signal, the
probability of matter outside the final compact object (HasRemnant) is
>99% [3].

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 [2],
 * bayestar.fits.gz,1, an initial localization generated by BAYESTAR [2].

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 4219 deg2.
Marginalized
over the whole sky, the a posteriori luminosity distance estimate is 278 +/-
134 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] Singer & Price PRD 93, 024013 (2016)
[3] Chatterjee et al. The Astrophysical Journal 896, 1 (2020)