At 16:00 UTC on the 18th of November, the fourth observing period (O4) of the LIGO–Virgo–KAGRA (LVK) Collaboration came to an end. Starting in May 2023, the LVK’s longest observing run yielded over 250 gravitational-wave candidates. Highlights published so far include the most massive merger seen to date, GW231123, a pair of events showing evidence of unusual and high black hole spins, GW241011 and GW241110 and a merger between a neutron star and a small black hole, GW230529. Upgrades performed before O4 brought the LVK detectors to their highest sensitivity yet, increasing the rate of gravitational-wave detection and allowing O4 to detect more than double the number of the three previous observation runs. O4 also saw the first time the four LIGO, Virgo and KAGRA detectors were observing simultaneously, marking a key milestone in gravitational-wave international collaboration! 

The LVK detected over 250 gravitational-wave candidates during O4, more than double the amount detected during the previous observing runs. Image courtesy of the LIGO-Virgo-KAGRA Collaboration.

Recently, the LVK released the data collected during the first period of O4 (O4a) in the fourth update to the Gravitational Wave Transient Catalogue (GWTC) and we can look forward to future updates covering the second and third period (O4b and c) in May and December 2026.  

When the LVK detectors will next be observing is currently uncertain, with an as-of-yet unnamed 6-month observing run set to take place sometime in the northern hemisphere summer/autumn of 2026, before we take a longer break as the detectors are upgraded in time to begin O5 in 2028, although this start date is currently being reassessed. The LVK keep their observing plan page updated with latest decisions. 

For now, keep an eye out for glitches from the last parts of O4c’s data, which will be uploaded to Gravity Spy soon!

Happy classifying 🕵️‍♀️, 

Elizabeth