The Cosmic-ray Knee as a Local Signature of Nearby PeVatrons

TL;DR

This paper investigates whether the cosmic-ray knee at 4 PeV is a global Galactic feature or a local signature from nearby sources, using simulations and observational comparisons.

Contribution

It introduces a simulation-based framework to distinguish between global and local origins of the cosmic-ray knee, highlighting the role of nearby PeVatrons.

Findings

A local knee is best explained by sources within ~1 kpc and 0.1-1 Myr old.

The Cygnus Cocoon is identified as a promising candidate for a nearby PeVatron.

Different regimes of PeV proton accelerator birth rates influence the knee's origin.

Abstract

A "knee" in the cosmic-ray spectrum, characterized by a sudden steepening of the spectral shape at $\sim 4$ PeV, may be interpreted either as a global feature of Galactic cosmic rays or as a local signature. In the former scenario, cosmic-ray spectra throughout the Galaxy would be similar to that observed in the solar neighborhood, and the knee would be a common feature of the cosmic-ray sea. In the latter scenario, the PeV cosmic-ray flux varies across the Galactic disk, and the knee is dominantly contributed by a small number of nearby sources. By simulating cosmic-ray propagation in the Galactic magnetic field and interstellar medium, we show that the two scenarios correspond to different regimes of the birth rate of PeV proton accelerators and depend on the presence of powerful nearby sources. By comparison with both cosmic-ray and gamma-ray observations, we find that a local knee would be best explained by sources located at distances of order $\sim1$ kpc and with ages in the range 0.1-1 Myr, with the Cygnus Cocoon being a particularly promising candidate.