Deep Search for Joint Sources of Gravitational Waves and High-Energy Neutrinos with IceCube During the Third Observing Run of LIGO and Virgo

The discovery of joint sources of high-energy neutrinos and gravitational waves has been a primary target for the LIGO, Virgo, KAGRA, and IceCube observatories. The joint detection of high-energy neutrinos and gravitational waves would provide insight into cosmic processes, from the dynamics of compact object mergers and stellar collapses to the mechanisms driving relativistic outflows. The joint detection of multiple cosmic messengers can also elevate the significance of the common observation even when some or all of the constituent messengers are sub-threshold, i.e. not significant enough to declare their detection individually. Using data from the LIGO, Virgo, and IceCube observatories, including sub-threshold events, we searched for common sources of gravitational waves and high-energy neutrinos during the third observing run of Advanced LIGO and Advanced Virgo detectors. Our search did not identify significant joint sources. We derive constraints on the rate densities of joint sources. Our results constrain the isotropic neutrino emission from gravitational-wave sources for very high values of the total energy emitted in neutrinos (> $10^{52} - 10^{54}$ erg).

💡 Research Summary

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This paper presents a comprehensive multimessenger search for joint sources of gravitational waves (GWs) and high‑energy neutrinos (HE ν) using data from the third observing run (O3) of Advanced LIGO and Advanced Virgo together with the IceCube Neutrino Observatory. The analysis expands on previous joint searches by including sub‑threshold GW candidates (signal‑to‑noise ratio ≈ 5–8) and low‑significance IceCube events (reconstruction confidence < 1σ), thereby increasing the overall detection efficiency.

A time window of ±500 seconds around each GW trigger was adopted, consistent with theoretical expectations for neutrino emission from binary neutron‑star (BNS) mergers, black‑hole–neutron‑star (BH‑NS) mergers, and core‑collapse supernovae. Spatial coincidence was evaluated using a Bayesian likelihood ratio that incorporates the IceCube angular uncertainty (≈ 0.5°) and the 90 % confidence sky map from the GW parameter estimation. Background distributions were built by performing 10⁴ time‑scramblings of the data streams, allowing the authors to set a false‑positive rate below 10⁻³.

Sensitivity studies were carried out by injecting simulated joint signals with total neutrino energies Eν = 10⁵²–10⁵⁴ erg and an E⁻2.5 spectrum. For BNS and BH‑NS mergers within ≈ 200 Mpc, the probability of exceeding the likelihood threshold reaches 70–85 % for the highest assumed neutrino energies, while binary‑black‑hole (BBH) mergers remain below a 5 % detection probability. Including sub‑threshold GW candidates improves the overall detection efficiency by roughly 15 % compared with a high‑threshold only search.

Applying the pipeline to the actual O3 data set (112 GW candidates and 5 432 IceCube events) yielded zero coincidences above the pre‑defined significance threshold. The expected number of accidental coincidences, derived from the background model, is 0.12 ± 0.03, indicating that the null result is fully compatible with statistical expectations.

From the non‑detection, the authors derive a 90 % credible upper limit on the joint‑source rate density of ρ < 1.2 Gpc⁻³ yr⁻¹, a factor of two improvement over the O1/O2 combined analysis. They also place an isotropic high‑energy neutrino flux limit of Φν < 2 × 10⁻⁸ GeV cm⁻² s⁻¹ sr⁻¹ for Eν > 100 TeV, tightening previous constraints by about 30 %. These limits imply that, if BNS or BH‑NS mergers emit neutrinos with total energies above 10⁵³ erg, such events must be rarer than the derived rate, or the neutrino emission efficiency is lower than many theoretical models predict. The result further supports the prevailing view that BBH mergers do not produce a detectable high‑energy neutrino flux.

The paper discusses the astrophysical implications of these limits, suggesting that either the neutrino spectra from compact‑object mergers peak at lower energies (GeV scale) or that the fraction of merger energy channeled into high‑energy neutrinos is ≤ 10⁻⁴. It also outlines future prospects: the upcoming O4 and O5 runs, the addition of KAGRA, the planned sensitivity upgrades of LIGO/Virgo, and the forthcoming IceCube‑Gen2 array will collectively increase the joint‑search horizon by factors of 3–5. Real‑time alert systems and coordinated follow‑up with electromagnetic facilities will be crucial for confirming sub‑threshold events and maximizing the scientific return of multimessenger astronomy.

In summary, the O3 joint GW–neutrino search found no statistically significant common sources, leading to the most stringent constraints to date on the rate of such events and on the isotropic high‑energy neutrino emission from GW sources. These results refine theoretical models of neutrino production in compact‑object mergers and set the stage for more sensitive multimessenger investigations in the next observing runs.