TITLE: GCN CIRCULAR NUMBER: 18330 SUBJECT: LIGO/Virgo G184098: Burst candidate in LIGO engineering run data DATE: 15/09/20 00:53:16 GMT FROM: Leo Singer at NASA/GSFC 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.]