Where Did the Amaterasu Particle Come From?

Abstract

The Telescope Array Collaboration recently reported the detection of a cosmic-ray particle, “Amaterasu,” with an extremely high energy of 2.4 × 1020 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 4π or 2726 deg2. 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 active galactic nuclei 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.

A nearby source of ultra-high energy cosmic rays

“…2 Just before the present paper was finished, a set of new regular GMF models was released [20]. Using results from another recent paper of the same authors [21] we estimated the magnitude of UHECR deflection in new models at the position of the highest energy TA event to be maximum a factor 1.3 of the deflection in JF12 GMF model. Using this estimation with our expected flux maps we found that the p-value for proton is less than 1% for new GMF models too.…”