Ultrahigh-Energy Cosmic Ray Composition from the Distribution of Arrival Directions

TL;DR

This study uses detailed simulations of galactic magnetic field effects on ultrahigh-energy cosmic rays to propose a new observational method for determining their nuclear composition, complementing existing techniques.

Contribution

It introduces a novel approach to identify UHECR composition by analyzing the energy-dependent shape of hot spots caused by magnetic deflections.

Findings

The 'cepa stratis' structure describes anisotropy patterns of UHECR nuclei.

Hot spots are elongated due to the regular structure of the Galactic magnetic field.

High-statistics data can reveal the nuclear composition through hot-spot contour analysis.

Abstract

The sources of ultrahigh-energy cosmic rays (UHECRs) have been difficult to catch. It was recently pointed out that while sources of UHECR protons exhibit anisotropy patterns that become denser and compressed with rising energy, nucleus-emitting-sources give rise to a cepa stratis (onion-like) structure with layers that become more distant from the source position with rising energy. The peculiar shape of the hot spots from nucleus-accelerators is steered by the competition between energy loss during propagation and deflection on the Galactic magnetic field (GMF). Here, we run a full-blown simulation study to accurately characterize the deflections of UHECR nuclei in the GMF. We show that while the cepa stratis structure provides a global description of anisotropy patterns produced by UHECR nuclei en route to Earth, the hot spots are elongated depending on their location in the sky due to the regular structure of the GMF. We demonstrate that with a high-statistics sample at the high-energy-end of the spectrum, like the one to be collected by NASA's POEMMA mission, the energy dependence of the hot-spot contours could become a useful observable to identify the nuclear composition of UHECRs. This new method to determine the nature of the particle species is complementary to those using observables of extensive air showers, and therefore is unaffected by the large systematic uncertainties of hadronic interaction models.