Studying the low-energy excess in cosmic-ray iron: a possible evidence of a massive supernova activity in the solar neighborhood via primary ⁶⁰Fe

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 Fe-56 isotope. Previously discovered were only deposits of radioactive Fe-60 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 Fe-60/Fe-56 ratio in the sources using Fe-60 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 similar to 10 TeV nucleon(-1) and the sub-Fe/Fe ratio calculated in the same framework.