Studying the low-energy excess in cosmic-ray iron: a possible evidence of a massive supernova activity in the solar neighborhood via primary 60Fe
July 01, 2021
Since its launch the Alpha Magnetic Spectrometer–02 (AMS-02) has delivered outstanding quality
measurements of the spectra of cosmic-ray (CR) species, which resulted in a number of breakthroughs.
A recently published spectrum of iron is of a particular interest. 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
inside the Local Bubble. Indeed, our analysis of AMS-02 measurements 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. This is the first time when the excess is found in the elemental CR iron that
is dominated by a stable 56Fe isotope. Previously discovered were only deposits of radioactive 60Fe
in terrestrial and lunar samples, and in CRs. To confirm the excess and clarify its nature, precise
measurements of other heavy CR species are of crucial importance. In this paper, we employ the
GalProp–HelMod framework to derive the 60Fe/56Fe ratio in the sources using 60Fe abundance
from ACE-CRIS and compare it with the SN yield. We also provide an updated local interstellar
spectrum (LIS) of iron in the energy range from 1 MeV nucleon−1 to ∼10 TeV nucleon−1 and the
sub-Fe/Fe ratio calculated in the same framework.
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