GCN Circular 27988

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
MS200618bx during real-time processing of data from LIGO
Hanford Observatory (H1) and LIGO Livingston Observatory
(L1) at 2020-06-18 12:18:10.329 UTC (GPS time:
1276517908.329). The candidate was found by the GstLAL
[1] analysis pipeline in the early warning configuration.
An Early Warning GCN Notice was issued 54 seconds before
the projected merger time. The GCN Notice included the
standard data products associated with CBC alerts, namely
localization [2] and source classification [3].

However, this Early Warning alert (MS200618bx) is no
longer of interest because it was not detected by the
regular, full-bandwidth search.

[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)