ANTARES & IceCube search for neutrinos from the Milky Way

[Published November 24th, 2018]


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 today in The Astrophysical Journal Letters (see here for the ArXiv version).

"This combination is an opportunity to benefit from the complementarity of the two detectors. While IceCube has much higher statistics, ANTARES offers enhanced sensitivity in the Southern sky where a larger flux is expected.", explains Timothée Grégoire, a main analyzer of the work, who recently defended his PhD at APC (Paris).

Combined upper limits (UL) at 90% confidence level (blue lines) on the three-flavor neutrino flux
of the KRAγ model with the 5 and 50PeV 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).
Taken from the ANTARES/IceCube paper.

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. 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 additional ANTARES and IceCube 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.