Properties of Iron Primary Cosmic Rays: Results from the Alpha Magnetic Spectrometer

We report the observation of new properties of primary iron (Fe) cosmic rays in the rigidity range 2.65 GV to 3.0 TV with $0.62\times {10}^6$ iron nuclei collected by the Alpha Magnetic Spectrometer experiment on the International Space Station. Above 80.5 GV the rigidity dependence of the cosmic ray Fe flux is identical to the rigidity dependence of the primary cosmic ray He, C, and O fluxes, with the $\mathrm{Fe}/\mathrm{O}$ flux ratio being constant at $0.155\pm 0.006$. This shows that unexpectedly Fe and He, C, and O belong to the same class of primary cosmic rays which is different from the primary cosmic rays Ne, Mg, and Si class.

Figure 1

(a) The AMS iron flux (red dots) and oxygen flux (violet triangles) multiplied by ${\tilde{R}}^{2.7}$ with total errors as a function of rigidity. (b) The AMS iron flux spectral index (red dots) and oxygen flux spectral index (violet triangles) dependence on rigidity. As seen, above 80.5 GV the rigidity dependence of the iron flux and spectral index follow the rigidity dependence of the oxygen flux and spectral index.

Figure 2

The AMS iron flux as a function of kinetic energy per nucleon $E_K$ multiplied by $E_K^{2.7}$ together with earlier measurements [6, 7, 9, 10, 11, 12, 13, 14, 15, 16]. For the AMS measurement $E_K=(\sqrt{Z^2{\tilde{R}}^2+M^2}-M)/A$ where $Z$, $M$, and $A$ are the ${}_{26}{}^{56}\mathrm{Fe}$ nuclear charge, mass, and atomic mass numbers, respectively.

Figure 3

The AMS $\mathrm{Fe}/\mathrm{O}$ ratio as a function of rigidity with total errors and with (a) red band ($1\sigma$) indicating the constant value fit ($\mathrm{Fe}/\mathrm{O}=0.155\pm 0.006$ with ${\chi }^2/\mathrm{d}.\mathrm{o}.\mathrm{f}.=8/11$) above 80.5 GV; b) dashed green curve, showing the prediction of the $\mathrm{Fe}/\mathrm{O}$ ratio by the GALPROP model [36] based on data available before AMS and dashed blue curve showing the prediction of the $\mathrm{Fe}/\mathrm{O}$ ratio by the latest GALPROP and HELMOD model [37] based on published AMS data without including data in this Letter; and (c) blue curve, showing the fit results with Eq. (3), and green band, showing the $\mathrm{Fe}/\mathrm{O}$ flux ratio at the source, i.e., fit results of Eq. (3) with $\lambda =0$.

Figure 4

The rigidity dependence of the Fe flux compared with the rigidity dependence of the He, C, and O fluxes and the Ne, Mg, and Si fluxes above 80.5 GV. For clarity, the He, O, Ne, and Si data points above 400 GV are displaced horizontally. For display purposes only, the He, C, O, Ne, Si, and Fe fluxes were rescaled as indicated. The shaded areas show the fit result of Eq. (5) of Ref. [3] to He, C, and O fluxes (magenta) and Ne, Mg, and Si fluxes (green).