Spectra of Cosmic-Ray Sodium and Aluminum and Unexpected Aluminum Excess

Abstract

Since its launch, the Alpha Magnetic Spectrometer-02 (AMS-02) has delivered outstanding quality measurements of the spectra of cosmic-ray (CR) species, p ¯ , e ±, and nuclei (H–Si, Fe), which resulted in a number of breakthroughs. The most recent AMS-02 result is the measurement of the spectra of CR sodium and aluminum up to ∼2 TV. Given their low solar system abundances, a significant fraction of each element is produced in fragmentations of heavier species, predominantly Ne, Mg, and Si. In this paper, we use precise measurements of the sodium and aluminum spectra by AMS-02 together with ACE-CRIS and Voyager 1 data to test their origin. We show that the sodium spectrum agrees well with the predictions made with the GalProp-HelMod framework, while the aluminum spectrum shows a significant excess in the rigidity range from 2–7 GV. In this context, we discuss the origin of other low-energy excesses in Li, F, and Fe found earlier. The observed excesses in Li, F, and Al appear to be consistent with the local Wolf-Rayet stars hypothesis, invoked to reproduce anomalous 22Ne/20Ne, 12C/16O, and 58Fe/56Fe ratios in CRs, while excess in Fe is likely connected with a past supernova activity in the solar neighborhood. We also provide updated local interstellar spectra (LIS) of sodium and aluminum in the rigidity range from a few megavolts to ∼2 TV. Our calculations employ the self-consistent GalProp-HelMod framework, which has proved to be a reliable tool in deriving the LIS of CR p ¯ , e −, and nuclei Z ≤ 28.

Spectra of Cosmic Ray Fluorine, Sodium and Aluminum and the Hints of Low-energy Excesses

“…predictions in some peculiar rigidity windows: contributions of local new sources are most likely. The WR hypothesis, that was invoked to reproduce the observed isotopic ratios ( 22 Ne/ 20 Ne, 12 C/ 16 O and 58 Fe/ 56 Fe), could be also consistent with the observed excesses in Li, F and Al, while excess in primary Fe should be connected with a past SN activity few Myr ago in the Local Bubble. Absence of a corresponding signal in sodium supports this scenario.…”

“…In particular, exploiting several of the most recent high quality AMS-02 results, we found that some CR ions still seem to require an additional primary component in some rigidity windows: they are lithium [2], iron [5], fluorine [6] and aluminum [7]. See [12,18,19,20] for insights and details.…”

“…Precise CR measurements over the last decade have lead to the discoveries of new features in spectra of CR species, various breaks and excesses in the energy range from sub-GeV to multi-TeV. Using combined measurements of AMS-02, Voyager 1, and ACE-CRIS we were able to establish a presence of primary lithium in CRs [12], to discover the low-energy excess in iron [18], a possible low-energy excess of fluorine [19] and a peculiar feature of the aluminum spectrum [20]. These nuclei show significant discrepancy w.r.t.…”

“…There are four possible physical reasons of the observed excess in the aluminum spectrum: (i) an incorrect spectrum of 28 Si, the major progenitor of 26,27 Al, (ii) errors in the total inelastic cross sections of Al, (iii) errors in the isotopic production cross sections of 26,27 Al, and (iv) an additional local component of primary Al. The first three can hardly account for such excess, as discussed in [20]. The latter is quite promising: observations of the distribution of the diffuse Galactic 1.809 MeV 𝛾-ray emission line from 26 Al decay by COMPTEL and INTEGRAL have shown that 26 Al nucleosynthesis is ongoing in the present Galaxy.…”

“…Apparently, the sources of cosmic Al are numerous, and are simultaneously also the sources of other rare isotopes, such as 7 Li, 19 F, 60 Fe and others [12,18,19]. Particularly interesting is a possible contribution of the massive WR stars proposed to explain the observed anomalous 22 Ne/ 20 Ne ratio and other observed ratios, 12 C/ 16 O, and 58 Fe/ 56 Fe, in CRs [10]. It was shown that all isotopic ratios measured with ACE-CRIS are consistent with CR source composition consisting of ∼20% of WR material mixed with ∼80% material with solar-system composition.…”