GCN Circular 24045
The LIGO Scientific Collaboration and the Virgo Collaboration report: Our third observing run ("O3") began as scheduled on 2019 April 1 at 15:00 UTC. At that time the LIGO Hanford, LIGO Livingston, and Virgo Observatories transitioned from engineering and commissioning to observing. All three detectors are operating at good sensitivity and stability. We are analyzing data in low latency and processing candidate transient events automatically. As of April 2 20:00 UTC, we have configured our low-latency analysis pipeline to send public alerts for significant gravitational-wave transient candidates that are detected in coincidence across two or more gravitational-wave detectors. Automated Preliminary GCN Notices will be sent immediately without any human intervention. Shortly afterward, they will be vetted by an LSC/Virgo rapid response team and either confirmed with an Initial GCN Notice and Circular, or withdrawn with a Retraction. Retraction notices may be issued more frequently over the next few weeks as our understanding of the instrumental background improves. For further information about vetting procedures, analysis methodology, and the contents of LIGO/Virgo public alerts, refer to the LIGO/Virgo Public Alerts User Guide <https://emfollow.docs.ligo.org/userguide/>. This marks the beginning of the era of public alerts for the field of gravitational-wave astronomy.