Intermediate mass and heavy Galactic cosmic-ray nuclei: the case of new AMS-02 measurements

TL;DR

This paper analyzes new AMS-02 measurements of cosmic ray nuclei, confirming the standard transport model for most nuclei but highlighting anomalies in iron at low rigidity, prompting further investigation.

Contribution

It demonstrates that the standard cosmic ray transport model explains most nuclei data without needing different injection spectra, except for iron at low rigidity.

Findings

Standard model fits lighter nuclei well

Iron nuclei show unusual low-rigidity trend

Discrepancies with previous low-energy measurements

Abstract

The recent measurement of the spectra of intermediate mass nuclei and iron nuclei carried out with the AMS-02 experiment provided us with the most complete set of data on cosmic ray fluxes to date, and allowed us to test the standard model for the transport of these particles through the Galaxy to the finest details. We show that the parameters derived from lighter primary and secondary elements in the cosmic radiation also lead to a good description of the data on heavier nuclei, with no need to invoke different injection spectra for such nuclei, provided the whole chain of fragmentation is properly accounted for. The only exception to this finding is represented by iron nuclei, which show a very unusual trend at rigidity $\lesssim 100$ GV. This trend reflects in a Fe/O ratio that is at odds with the results of the standard model of cosmic ray transport, and is in contradiction with data collected by HEAO, ACE-CRIS and Voyager at lower energy. We speculate on possible origins of such findings.