GCN Circular 18330

Dear colleagues,

We would like to bring to your attention a trigger identified by the
online Burst analysis during the ongoing Engineering Run 8 (ER8).
Normally, we would send this in the form of a private GCN Circular, but
the LIGO/Virgo GCN Circular list is not ready yet.

The LIGO Scientific Collaboration and Virgo report that the cWB unmodeled
burst analysis identified candidate G184098 during real-time processing of
data from LIGO Hanford Observatory (H1) and LIGO Livingston Observatory
(L1) at 2015-09-14 09:50:45 UTC (GPS time: 1126259462.3910). Alerts were
not sent in real-time because the candidate occurred in ER8 data; however,
we have now sent GCN notices through our normal channel.

G184098 is an unvetted event of interest, as the false alarm rate (FAR)
determined by the online analysis would have passed our stated alert
threshold of ~1/month. The event's properties can be found at this URL:

https://gracedb.ligo.org/events/G184098

There are important caveats associated to this event:
 * It occurred before the initiation of the planned observing run;
 * The detectors were not in their final O1 configuration;
 * Calibration is not finalized.
In particular, calibration uncertainties may imply systematic errors in
sky localization.

Nevertheless, the trigger is of sufficient interest to present an
important opportunity to exercise the EM follow-up process, and we invite
you to make use of the information in the above link.

Two sky maps are available at the moment: the rapid localization from cWB
itself, and a refined localization from LALInference Burst (LIB). They are
in good qualitative agreement with each other. The 50% credible region
spans about 200 deg2 and the 90% region about 750 deg2.

Updates on our analysis of this event will be sent as they become
available.

***PLEASE REMEMBER*** that this message is being sent only to groups that
have signed MOUs with LIGO and Virgo and have observing capabilities
during O1, and that this event and the LIGO/Virgo data related to it
remain confidential at this time.

We are looking forward to learn what you will see, and to an exciting time
in the Advanced LIGO/Virgo era.

Best wishes,
Leo, Marica, Peter

[GCN OPS NOTE(19sep15): This Circular was originally published on 05:39 16-Sep-2015 UT.]