Where Did the Amaterasu Particle Come From?

TL;DR

This paper investigates the origin of the ultra-high-energy cosmic ray particle 'Amaterasu', analyzing its possible source location, composition, and the astrophysical objects involved, using magnetic field modeling and sky mapping.

Contribution

It provides the first detailed localization analysis of the Amaterasu event, estimating its source region and evaluating potential astrophysical sources with updated magnetic field models.

Findings

Estimated source region of 2726 deg².

Maximum source distance of 8-50 Mpc.

No candidate sources among powerful radio galaxies.

Abstract

The Telescope Array Collaboration recently reported the detection of a cosmic-ray particle, "Amaterasu", with an extremely high energy of $2.4\times10^{20}$ eV. Here we investigate its probable charge and the locus of its production. Interpreted as a primary iron nucleus or slightly stripped fragment, the event fits well within the existing paradigm for UHECR composition and spectrum. Using the most up-to-date modeling of the Galactic magnetic field strength and structure, and taking into account uncertainties, we identify the likely volume from which it originated. We estimate a localization uncertainty on the source direction of 6.6\% of $4\pi$ or 2726 deg$^2$. The uncertainty of magnetic deflections and the experimental energy uncertainties contribute about equally to the localization uncertainty. The maximum source distance is 8-50 Mpc, with the range reflecting the uncertainty on the energy assignment. We provide sky maps showing the localization region of the event and superimpose the location of galaxies of different types. There are no candidate sources among powerful radio galaxies. An origin in AGNs or star-forming galaxies is unlikely but cannot be completely ruled out without a more precise energy determination. The most straightforward option is that Amaterasu was created in a transient event in an otherwise undistinguished galaxy.