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

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 $Z\le 28$.

12 pages, 9 figures, 5 tables; Accepted for publication in the Astrophysical Journal. arXiv admin note: text overlap with arXiv:2006.01337

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• The rigidity dependence of galactic cosmic-ray fluxes and its connection with the diffusion coefficient
• Proposed Lunar Measurements of $r$-Process Radioisotopes to Distinguish Origin of Deep-sea 244Pu
• Current status and desired accuracy of the isotopic production cross-sections relevant to astrophysics of cosmic rays II. Fluorine to Silicon (and updated LiBeB)
• Injection spectra of different species of cosmic rays from AMS-02, ACE-CRIS and Voyager-1
• Very Local Impact on the Spectrum of Cosmic-Ray Nuclei below 100 TeV
• Transport parameters from AMS-02 F/Si data and fluorine source abundance
• The GALPROP Cosmic-ray Propagation and Non-thermal Emissions Framework: Release v57
• Precision spectral measurements of Chromium and Titanium from 10 to 250 GeV$/n$ and sub-Iron to Iron ratio with the Calorimetric Electron Telescope on the ISS
• Spectra of He isotopes and the ${}^3$He/${}^4$He ratio