One-point fluctuation analysis of IceCube neutrino events outlines a significant unassociated isotropic component and constrains the Galactic contribution

The origins of the extraterrestrial neutrinos observed in IceCube have yet to be determined. In this study we perform a one-point fluctuation analysis of the six-year high-energy starting event shower data, with fixed non-Poissonian contributions from atmospheric, galactic and some extragalactic components, as well as an isotropic (and weakly non-Poissonian) template. In addition to the star-forming galaxies and blazars, our analysis suggests the presence of an additional isotropic component, not associated with any detected class of point sources, with best-fit intensity of (2.8 ± 0.2)×10⁻¹⁸(E/100  TeV)^−2.7±0.5 cm⁻² s⁻¹ sr⁻¹ GeV⁻¹. For the first time, we also consider high-energy extrapolations of several phenomenological models for the diffuse galactic emission (tuned to both local cosmic-ray data and diffuse gamma-ray emission in the GeV–TeV domain). We demonstrate the potential of our framework in discriminating between different scenarios, with possible implications on the physics of cosmic-ray transport in the TeV–PeV range.