What have we learned about the sources of ultrahigh-energy cosmic rays via neutrino astronomy?

TL;DR

This paper reviews neutrino observations to understand the sources of ultrahigh-energy cosmic rays, highlighting constraints on known astronomical objects and implications for cosmic-ray origin theories.

Contribution

It synthesizes recent neutrino data to evaluate and constrain models of extragalactic cosmic-ray sources, emphasizing the limitations of known objects like AGNs and GRBs.

Findings

Extragalactic sources with optical depth > 0.01 contribute significantly to cosmic rays at 10 PeV.

Neutrino flux limits challenge the role of AGNs and GRBs as sources of the highest-energy cosmic rays.

Certain models of cosmic-ray origins are becoming less plausible due to neutrino observation constraints.

Abstract

Observations of TeV--PeV-energy cosmic neutrinos by the IceCube observatory have suggested that extragalactic cosmic-ray sources should have an optical depth greater than $\sim$0.01 and contribute to more than 10\% of the observed bulk of cosmic rays at 10 PeV. If the spectrum of cosmic rays from these extragalactic sources extends well beyond 1 EeV, the neutrino flux indicates that extragalactic cosmic-ray protons are dominant in the observed total cosmic-ray flux at 1 EeV. Among known powerful astronomical objects, including gamma-ray bursters (GRBs), only flat-spectrum radio quasars could (barely) satisfy these conditions. On the other hand, the null detection of neutrinos with energies well beyond PeV has excluded the possibility that radio-loud active galactic nuclei (AGNs) and/or GRBs, the popular source candidates discussed in the literature, are the origins of the highest-energy cosmic rays ($\sim100 {\rm EeV}$) if they are composed mainly of protons. Their origins must be objects that have evolved on time scales comparable to or slower than the star formation rate. These considerations indicate that none of the known extragalactic astronomical objects can be simultaneously a source of both PeV- and trans-EeV-energy cosmic rays. As a result of the stringent limits on EeV-energy neutrino fluxes, a significant part of the parameter space for the AGN and new-born pulsar models is starting to seem unfavorable, even for scenarios of mixed and heavy cosmic-ray compositions at the highest energies.