ER15 started at 1500 UTC on April 26, 2023; we plan to start O4 on 24 May 2023.
LIGO Hanford and LIGO Livingston joined on Apr/26 at 1500 UTC with plans to perform calibration measurements and other tests.
Virgo joined on Apr/26 at 1500 UTC, for a limited fraction of time over the duration of the ER, giving priority to commissioning activities aiming at improving the sensitivity
KAGRA will continue commissioning up to one week before the start of the O4 to improve sensitivity.