Indications of Negative Evolution for the Sources of the Highest Energy Cosmic Rays

TL;DR

Recent measurements suggest that the sources of the highest energy cosmic rays predominantly emit intermediate mass nuclei and are more likely located in the low-redshift universe, with models favoring negative source evolution.

Contribution

This study provides evidence that the sources of ultra-high energy cosmic rays mainly emit intermediate mass nuclei and are likely to be situated in the low-redshift universe, favoring models with negative evolution.

Findings

Data favors models with intermediate mass nuclei as primary cosmic ray sources.

Hard spectral index ($pprox;1$) required if source density does not evolve with redshift.

Models with negative source evolution are consistent with softer spectra ($pprox;2$).

Abstract

Using recent measurements of the spectrum and chemical composition of the highest energy cosmic rays, we consider the sources of these particles. We find that the data strongly prefers models in which the sources of the ultra-high energy cosmic rays inject predominantly intermediate mass nuclei, with comparatively few protons or heavy nuclei, such as iron or silicon. If the number density of sources per comoving volume does not evolve with redshift, the injected spectrum must be very hard ($\alpha\simeq 1$) in order to fit the spectrum observed at Earth. Such a hard spectral index would be surprising and difficult to accommodate theoretically. In contrast, much softer spectral indices, consistent with the predictions of Fermi acceleration ($\alpha\simeq 2$), are favored in models with negative source evolution. With this theoretical bias, these observations thus favor models in which the sources of the highest energy cosmic rays are preferentially located within the low-redshift universe.