Role of Galactic Sources and Magnetic Fields in Forming the Observed Energy-Dependent Composition of Ultrahigh-Energy Cosmic Rays

Recent results from the Pierre Auger Observatory, showing energy-dependent chemical composition of ultrahigh-energy cosmic rays (UHECRs) with a growing fraction of heavy elements at high energies, suggest a possible non-negligible contribution of the Galactic sources. We show that, in the case of UHECRs produced by gamma-ray bursts or rare types of supernova explosions that took place in the Milky Way in the past, the change in UHECR composition can result from the difference in diffusion times for different species. The anisotropy in the direction of the Galactic center is expected to be a few per cent on average, but the locations of the most recent or closest bursts can be associated with observed clusters of UHECRs.

Figure 1

Predicted UHECR spectra, assuming that the sources produce 90% protons and 10% iron, with identical spectra $\propto E^{-2.3}$, and that the source distribution traces the distribution of stars in the Galaxy. We used samples of ${10}^3$ GRBs at random locations with time intervals of ${10}^5$ years. The magnetic field was assumed to be $4\mu \mathrm{G}$, coherent over $l_0=0.2\mathrm{kpc}$ domains, ${\delta }_1=0.3$, ${\delta }_2=0$. The overall power and the iron fraction were adjusted to fit the PAO data points [24] (shown). For each random sample, the fit parameters differ slightly, depending on the location of the latest or closest burst. To model the spectrum at $E>3\times {10}^{19}\mathrm{eV}$, one has to account for energy losses and (proton) contribution of extragalactic sources, which we leave for future work.

Figure 2

Galactocentric anisotropy for a source distribution that traces the stellar counts in the MW, modeled by random generation of ${10}^3$ bursts separated by time intervals of ${10}^5\mathrm{yr}$. The model parameters are the same as in Fig. 1. Although the anisotropy in protons is large at high energies, their contribution to the total flux is small, so the total anisotropy $<10\%$, consistent with the observations. The latest GRBs do not introduce a large degree of anisotropy, as it would be in the case of UHE protons, but they can create “hot spots” and clusters of events.