Study of Galactic interactions and propagation properties of ultrahigh energy cosmic rays using the precision spectrum from the Pierre Auger Observatory

The Auger Collaboration has recently published the precise energy spectrum of cosmic rays above 1 EeV, which exhibits rich and interesting features. These features provide an opportunity to investigate the Galactic propagation and interaction of ultrahigh energy cosmic rays (UHECRs). While the classic dip model, which describes the UHECR propagation and interaction with cosmic microwave background in extragalactic space by theoretical calculation, well explains the cutoff behavior of the highest high energy spectrum of Auger, it fails to describe the low energy features. It indicates that the Galactic propagation and interaction of UHECRs might play an important role in modulating the low energy side of the Auger spectrum. With the disk-halo diffusion model, this work demonstrates excellent agreement with the energy spectrum measured by Auger and supports that the diffusion coefficient follows a power law with an index of 1.81_-0.04+0.05 in the Galaxy, at least above 5×10¹⁸ eV. The inferred diffusion coefficient is 1 to 2 orders of magnitude smaller than these extrapolated from low energy studies when conventional model parameters are adopted. The study of the modulation effect at a cluster of the Galaxy level might be needed.