The IceCube observatory in Antarctica checked whether gravitational waves are accompanied by a burst of high-energy neutrinos. No significant signal was found, but the number of analyzed sources nearly doubled compared to all previous observing runs combined.
Gravitational waves are produced when black holes and neutron stars merge. During such a merger, particle acceleration is expected, and with it the production of neutrinos (nearly massless particles that travel through space undeflected and barely interacting with anything). This is why neutrinos can be traced back to their source and help pinpoint where a gravitational wave came from.
The LIGO, Virgo, and KAGRA detectors record gravitational waves and quickly alert astronomers. For the O4a run, the IceCube team set up an automated pipeline that receives these alerts, analyzes the data, and sends results to the community. This allowed a much faster response than in the previous run, O3.
Two methods were used: an unbinned maximum likelihood analysis for significant alerts and a Bayesian analysis that accounts for prior information about sources. The most notable neutrino–gravitational-wave coincidence involved candidate S231025a, with neutrinos within ±500 seconds of the merger overlaid on the map of the source's likely location.
A faster response matters for finding the so-called third messenger. When IceCube immediately points out a direction, other telescopes narrow the patch of sky to search for an electromagnetic counterpart. With the planned IceCube-Gen2 expansion, such joint observations should become more frequent.