Gravitational waves (GWs) are produced by some of the most violent and energetic astrophysical phenomena, such as black hole and neutron star mergers. During the merger of two black holes or neutron stars, particle acceleration is expected to occur. This can produce a short gamma-ray burst as well as neutrinos, cosmic messengers hurtling through space unimpeded. Because of this, neutrinos can be traced back to their sources, helping pinpoint GW sources that are otherwise difficult to locate using GW data alone. The observation of high-energy neutrinos from one of these events can provide valuable information to other telescopes looking for GW events and point to an astrophysical source of GWs, light, and neutrinos.
In a study submitted to The Astrophysical Journal, the IceCube Collaboration presents a real-time search for the joint detection of GWs and high-energy neutrinos. The result was not statistically significant and upper limits were set for more astrophysical sources, almost doubling the number of sources analyzed compared to all of the previous observing runs combined.
The ground-based GW detectors, LIGO, Virgo, and KAGRA (LVK), are able to observe GW events and alert the astrophysics community very shortly after they happen. Previously, the IceCube Collaboration looked for joint emission of GWs and high-energy neutrinos with IceCube data and data from the first two and the third observing runs of the LIGO and Virgo detectors. For this analysis, researchers used data from O4a, the first part of the most recent observing run, O4.
“For this LVK observing run, we set up an automated pipeline for receiving, analyzing, and sending results to the astronomical community,” explains IceCube collaborator and study lead Jessie Thwaites, a postdoctoral researcher at Queen’s University. “This allowed us to improve our response time to these alerts.”
Thwaites conducted the study while a PhD student at the University of Wisconsin–Madison (UW–Madison), along with study coleads Doğa Veske (Middle East Technical University/Heidelberg University/Columbia University), Justin Vandenbroucke (UW–Madison), and Zsuzsa Marka (Columbia University).
“Real-time searches are crucial for discovering the next multimessenger detection. Our follow-ups shrink the area that needs to be scanned by astronomers, making feasible follow-ups for finding the third messenger,” says the study lead Veske, now an assistant professor at Middle East Technical University.
They used two methods, both of which were previously used to search for neutrino emission associated with GWs: an unbinned maximum likelihood analysis on significant alerts and a Bayesian analysis accounting for prior information about astrophysical sources on both significant and low-significance alerts.
“In this run, we were able to send our results much faster than in previous real-time runs, which improves our chances of observing one of these multimessenger sources with many telescopes,” says Thwaites.
“In response to our alerts, other telescopes pointed in the direction of the candidate neutrinos that we detected to search for electromagnetic counterparts,” adds Vandenbroucke.
With the planned improvements of the GW detectors and the proposed expansion of IceCube, IceCube-Gen2, scientists will be able to see more merger events and the neutrinos that are possibly produced in them.
+ info “IceCube Real-time Searches for High-energy Neutrinos Coincident with LIGO/Virgo/KAGRA Gravitational Wave Alerts in O4a,” IceCube Collaboration: R. Abbasi et al. Submitted to The Astrophysical Journal. arxiv.org/abs/2606.13762v1