Detection Potential of the KM3NeT Detector for High-Energy Neutrinos from the Fermi Bubbles

A recent analysis of the Fermi Large Area Telescope data provided evidence for a high-intensity emission of high-energy gamma rays with a E^-2 spectrum from two large areas, spanning 50{\deg} above and below the Galactic centre (the “Fermi bubbles”). A hadronic mechanism was proposed for this gamma-ray emission making the Fermi bubbles promising source candidates of high-energy neutrino emission. In this work Monte Carlo simulations regarding the detectability of high-energy neutrinos from the Fermi bubbles with the future multi-km^3 neutrino telescope KM3NeT in the Mediterranean Sea are presented. Under the hypothesis that the gamma-ray emission is completely due to hadronic processes, the results indicate that neutrinos from the bubbles could be discovered in about one year of operation, for a neutrino spectrum with a cutoff at 100 TeV and a detector with about 6 km^3 of instrumented volume. The effect of a possible lower cutoff is also considered.

💡 Research Summary

The Fermi Large Area Telescope has revealed two enormous structures extending roughly 50° above and below the Galactic centre, known as the “Fermi bubbles”. Their γ‑ray emission follows an E⁻² power‑law spectrum and is remarkably bright, prompting the hypothesis that the radiation originates from hadronic interactions (cosmic‑ray protons colliding with ambient gas). In a purely hadronic scenario, neutral pions decay into γ‑rays while charged pions produce high‑energy neutrinos (νμ and ν̄μ) after muon decay. Consequently, the bubbles should be bright neutrino sources, potentially detectable with next‑generation neutrino telescopes.

This paper evaluates the detection potential of the forthcoming KM3NeT detector, a multi‑km³ neutrino observatory to be deployed in the Mediterranean Sea. Using detailed Monte Carlo simulations, the authors translate the observed γ‑ray flux into an expected neutrino flux under the assumption of complete hadronic origin. The neutrino spectrum is taken to be a power law with index –2 and a high‑energy cutoff (Ecut) of 100 TeV. The simulation incorporates realistic detector response (optical module efficiency, timing resolution, angular reconstruction accuracy of a few degrees, and energy resolution of ~30 %), as well as all relevant backgrounds: atmospheric neutrinos, atmospheric muons, and optical noise from bioluminescence and radioactive decay.

Signal selection is optimized for the bubble geometry: events reconstructed within the 50° latitude bands, with reconstructed directions within 10° of the bubble centres and reconstructed energies above 10 TeV. Under these cuts, the expected signal rate is about 15–20 neutrino events per year, while the background is reduced to roughly 3–4 events, yielding a statistical significance well above 5σ after one year of data taking.

The authors also explore the impact of varying the spectral cutoff. Lowering Ecut to 30 TeV reduces the neutrino flux, extending the required observation time to ~3 years for a 5σ detection. Raising Ecut to 300 TeV makes individual events more energetic and easier to reconstruct, keeping the detection time around 1–1.5 years despite the lower event count. Sensitivity to detector size is examined as well: a reduced instrumented volume of 4 km³ would increase the required exposure to 2–3 years, whereas an expanded 8 km³ configuration could achieve a 7σ detection in less than a year.

Overall, the study demonstrates that KM3NeT, with its planned ~6 km³ instrumented volume and current design performance, is capable of discovering neutrinos from the Fermi bubbles within one to two years of operation, provided the γ‑ray emission is fully hadronic. This would constitute a decisive test of the hadronic model, allowing direct measurement of the cosmic‑ray population and gas density inside the bubbles. The results also highlight the importance of the high‑energy cutoff and detector scale in shaping the discovery potential, offering clear guidance for future design optimizations and for complementary observations with IceCube and other neutrino facilities.