Are high-energy neutrinos also produced in the Milky Way?

The first measurement of the astrophysical neutrino flux in IceCube showed a clear isotropic, thus extragalactic, component. However, there was also a hint of a correlation with the galactic plane, suggesting as much as 50% of the flux comes from our own galaxy. As more neutrinos were detected and the search extended to other techniques, the hypothesis that some of these neutrinos are of galactic origin has been gaining adherents.

This week, the IceCube Collaboration presents a new search for neutrino emission associated with the galactic plane with seven years of IceCube data. The results, submitted to The Astrophysical Journal, are not conclusive but set new constraints on the possible galactic contribution.

7years Galactic Plane
Upper limits at the 90% confidence level on the three-flavor neutrino flux from the Galaxy with respect to KRA model predictions and the total measured astrophysical flux. Credit: IceCube Collaboration.

Gamma-ray telescopes have observed diffuse emission from our galaxy produced by the interaction of cosmic rays with interstellar gas. The detection of this gamma-ray flux, which aligns with the Milky Way plane and is due to the decay of neutral pions, implies the existence of a similar neutrino flux via the decay of charged pions. On top of this cosmic-ray induced flux, neutrinos from discrete sources in our galaxy could also contribute to neutrino emission from the galactic plane.

The latest gamma-ray data from Fermi-LAT, H.E.S.S, and Milagro suggest that the neutrino flux from cosmic-ray interactions in the galaxy is higher than previous models predicted. “Our sensitivity in neutrinos is now comparable to the latest model predictions,” says Jon Dumm, one of the lead authors of this work and an IceCube researcher at Stockholm University. “We cannot exclude the new generation of models because we see a small though nonsignificant excess in the neutrino data.”

Previous searches in IceCube used a class of events starting inside the detector, and while this selection gives a very pure sample of astrophysical neutrinos, it’s also very restrictive and cuts many interesting events. “This new analysis focuses on muon neutrinos. We are primarily sensitive in the Northern Hemisphere, away from the galactic center, but we have very high statistics,” explains Dumm. This large number of neutrino candidates has allowed IceCube scientists to constrain the galactic flux to be less than about 14% of the total flux. The key assumption is that the high-energy signal we see above ~10 TeV extends downward another order of magnitude in energy where the new analysis is sensitive.

“We have cross-checked these results with a second analysis similar to our searches for neutrino emission anywhere in the northern sky,” says Christian Haack, PhD student at RWTH-Aachen University. “And we have shown that the measured isotropic astrophysical muon neutrino flux is robust against the presence of a subdominant flux from the galactic plane.”

The next steps in the search for galactic neutrinos will be to merge this new analysis with other detection channels inside IceCube, such as cascade-like events, and with data from the ANTARES neutrino detector. This has the potential to double the sensitivity in the near future and, hopefully, make a more decisive statement about neutrinos from the Milky Way. Dumm remains optimistic: “It may take several years or a new generation of neutrino detectors, but neutrinos from the Milky Way will be discovered, and with them, a new view of particle acceleration and propagation in our galaxy.”

+ info “Constraints on Galactic Neutrino Emission with Seven Years of IceCube Data,” The IceCube Collaboration: M. G. Aartsen et al. The Astrophysical Journal 849 (2017) 67,