Hint for a TeV neutrino emission from the Galactic Ridge with ANTARES

Abstract

Interactions of cosmic ray protons, atomic nuclei, and electrons in the interstellar medium in the inner part of the Milky Way produce a $\gamma$-ray flux from the Galactic Ridge. If the $\gamma$-ray emission is dominated by proton and nuclei interactions, a neutrino flux comparable to the $\gamma$-ray flux is expected from the same sky region. Data collected by the ANTARES neutrino telescope are used to constrain the neutrino flux from the Galactic Ridge in the 1-100 TeV energy range. Neutrino events reconstructed both as tracks and showers are considered in the analysis and the selection is optimized for the search of an excess in the region $|l|<30°$, $|b|<2°$. The expected background in the search region is estimated using an off-zone region with similar sky coverage. Neutrino signal originating from a power-law spectrum with spectral index ranging from ${\Gamma }_{\nu }=1$ to $4$ is simulated in both channels. The observed energy distributions are fitted to constrain the neutrino emission from the Ridge. The energy distributions in the signal region are inconsistent with the background expectation at $\sim 96\%$ confidence level. The mild excess over the background is consistent with a neutrino flux with a power law with a spectral index $2.45_{-0.34}^{+0.22}$ and a flux normalization $d{N}_{\nu }/d{E}_{\nu }=4.0_{-2.0}^{+2.7}\times 10^{-16}{\text{GeV}}^{-1}{\text{cm}}^{-2}{\text{s}}^{-1}{\text{sr}}^{-1}$ at 40 TeV reference energy. Such flux is consistent with the expected neutrino signal if the bulk of the observed $\gamma$-ray flux from the Galactic Ridge originates from interactions of cosmic ray protons and nuclei with a power-law spectrum extending well into the PeV energy range.

8 pages, 5 figures

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