A view of the Milky Way with IceCube and ANTARES

Cosmic rays are charged particles and therefore easily interact with matter and magnetic fields as they travel from their sources to Earth. In the Milky Way, these high-energy particles are expected to interact with the interstellar gas and radiation fields, producing a diffuse flux of neutrinos in the galactic plane. In a similar way, these interactions would produce diffuse high-energy gamma-ray emission, which has been measured by the Fermi Large Area Telescope (Fermi LAT).

Our knowledge of cosmic-ray production and diffusion mechanisms is still very limited. Neutrinos allow us to test our models at higher energies than do gamma rays. In a first-time effort to combine IceCube and ANTARES data to constrain galactic cosmic-ray models, scientists from both collaborations have set new limits on some of these models as well as a new limit for the galactic contribution to the IceCube neutrino flux. These results have been published this week in the journal Astrophysical Journal Letters

Antares IceCube
Combined upper limits (UL) at 90\% confidence level (blue lines) on the three-flavor neutrino flux with 5 and 50 PeV cutoffs (black lines). The boxes represent the diffuse astrophysical neutrino fluxes measured by IceCube using an isotropic flux template with starting events (yellow) and upgoing tracks (green).

“The idea is simple enough: galactic cosmic rays are accelerated in sources like supernova remnants distributed throughout the galaxy. Then, they propagate in the interstellar medium producing gamma rays and neutrinos as they interact with gas and radiation fields around them,” explains Christian Haack, a main analyzer of this work and a PhD candidate at RWTH Aachen.

The observed gamma-ray emission is well explained up to 10 GeV by conventional models that calculate the propagation of cosmic rays and the related diffuse gamma-ray and neutrino emission under the hypothesis of uniform cosmic-ray diffusion.

Above these energies, models that parametrize the diffusion with a radial dependence predict a spectral hardening in the galactic center, or more specifically the so-called galactic ridge—the intersection of the galactic center with the galactic plane—that agrees with observed gamma-ray emission up to a few hundred GeV.

Milky Way neutrino
Neutrino flux per unit of solid angle shown as a function of direction in equatorial coordinates (Hammer projection).

Radial models also predict enhanced gamma-ray and neutrino emission at higher and higher energies in the southern sky, which in IceCube results in up to twice as many galactic astrophysical neutrinos in the southern sky than those coming from the north.

In this analysis, researchers have combined seven years of IceCube track data—mainly astrophysical muon neutrinos—with ten years of ANTARES track and shower data—neutrinos of all flavors—to perform the most rigorous test of the existing galactic cosmic-ray production and transport models.

Using an energy cut-off of 5 PeV––this is the maximum energy of galactic cosmic rays that also determines secondary gamma-ray and neutrino emission––results were found to be in good agreement with the predictions of the CR model and are still far from setting any constraints on the diffusion mechanisms and/or the maximum energy of galactic cosmic rays. Using this model, scientists have estimated that the galactic contribution to the IceCube astrophysical neutrino flux cannot be larger than 8.5%.

In the future, the inclusion of IceCube shower data and increased data sets will improve the sensitivity of these measurements, allowing stronger tests of the cosmic-ray production and diffusion models, thus telling us more about the processes that power sources like supernova remnants as well as the distribution of interstellar gas and radiation fields.

+ info “Joint Constraints on Galactic Diffuse Neutrino Emission from the ANTARES and IceCube Neutrino Telescopes,” The ANTARES and IceCube Collaborations: A. Albert et al. The Astrophysical Journal Letters 868 (2018), arxiv.org/abs/1808.03531.