Pinning down the cosmic ray source mechanism with new IceCube data

TL;DR

This paper analyzes recent IceCube neutrino data, testing an unbroken power-law spectrum hypothesis, and explores implications for cosmic ray sources and features like the knee and ankle in the spectrum.

Contribution

It assesses the compatibility of IceCube data with a specific neutrino spectrum and proposes methods to distinguish Galactic source models based on energy transfer and spectral features.

Findings

Spectral index ~ 2.3 fits data at 1.5σ level

Galactic sources could explain observed neutrino flux

Future data will test the unbroken power-law hypothesis

Abstract

Very recently the IceCube Collaboration has reported an observation of 28 neutrino candidates with energies between 50 TeV and 2 PeV, constituting a 4.1$\sigma$ excess compared to the atmospheric background. In this article we investigate the compatibility between the data and a hypothesized unbroken power-law neutrino spectrum for various values of spectral index \Gamma >= 2. We show that \Gamma ~ 2.3 is consistent at the ~ 1.5$\sigma$ level with the observed events up to 2 PeV and to the null observation of events at higher energies. We then assume that the sources of this unbroken spectrum are Galactic, and deduce (i) an energy-transfer fraction from parent protons to pions, and (ii) a way of discriminating among models which have been put forth to explain the "knee" and "ankle" features of the cosmic ray spectrum. Future IceCube data will test the unbroken power law hypothesis and provide a multi-messenger approach to explaining features of the cosmic ray spectrum, including the transition from Galactic to extragalactic dominance.