A discovery of a low-energy excess in cosmic-ray iron: an evidence of the past supernova activity in the Local Bubble

TL;DR

This paper reports a low-energy excess in cosmic-ray iron spectra, suggesting recent local supernova activity, supported by data from AMS-02, Voyager 1, and ACE-CRIS, and consistent with radioactive $^{60}$Fe deposits.

Contribution

It identifies a new low-energy feature in cosmic-ray iron spectra and links it to recent local supernova activity, providing an updated local interstellar spectrum using the GalProp-HelMod framework.

Findings

Discovery of a bump in iron spectrum at 1-2 GV

Correlation of the excess with recent $^{60}$Fe deposits

Support for local supernova activity as the source

Abstract

Since its launch, the Alpha Magnetic Spectrometer - 02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species, $\bar{p}$, $e^{\pm}$, and nuclei, $_1$H-$_8$O, $_{10}$Ne, $_{12}$Mg, $_{14}$Si, which resulted in a number of breakthroughs. One of the latest long awaited surprises is the spectrum of $_{26}$Fe just published by AMS-02. Because of the large fragmentation cross section and large ionization energy losses, most of CR iron at low energies is local, and may harbor some features associated with relatively recent supernova (SN) activity in the solar neighborhood. Our analysis of new iron spectrum together with Voyager 1 and ACE-CRIS data reveals an unexpected bump in the iron spectrum and in the Fe/He, Fe/O, and Fe/Si ratios at 1-2 GV, while a similar feature in the spectra of He, O, Si, and in their ratios is absent, hinting at a local source of low-energy CRs. The found excess fits well with recent discoveries of radioactive $^{60}$Fe deposits in terrestrial and lunar samples, and in CRs. We provide an updated local interstellar spectrum (LIS) of iron in the energy range from 1 MeV nucleon$^{-1}$ to $\sim$10 TeV nucleon$^{-1}$. Our calculations employ the GalProp-HelMod framework that is proved to be a reliable tool in deriving the LIS of CR $\bar{p}$, $e^{-}$, and nuclei $Z\le28$.