On the excess of ultra-high energy cosmic rays in the direction of Centaurus A

TL;DR

This study investigates the excess of ultra-high energy cosmic rays near Centaurus A, suggesting they are likely intermediate-mass nuclei accelerated by stellar winds in star-forming regions, with implications for cosmic ray origin and composition.

Contribution

The paper combines anisotropy analysis with propagation simulations to propose a plausible origin of cosmic rays from Centaurus A involving intermediate-mass nuclei and stellar wind acceleration.

Findings

Only particles with charge Z ≤ 10 can stay within the 18° window of Centaurus A.

Heavy composition at high energies suggests intermediate-mass nuclei as primary cosmic rays.

Acceleration in stellar winds, not lobes, is a plausible source for the excess cosmic rays.

Abstract

A posteriori anisotropy study of ultra-high energy cosmic rays (UHECRs) with the Pierre Auger Observatory (PAO) has shown evidence of excess of cosmic ray particles above 55 EeV within $18^{\circ}$ of the direction of the radio galaxy Centaurus A. However, the origin of the excess remains elusive. We simulate the propagation of different species of particles coming from the direction of Centaurus A in the Galactic magnetic fields, and find that only particles of nuclear charge $Z\la 10$ can avoid being deflected outside of the $18^{\circ}$ window of Centaurus A. On the other hand, considering the increasingly heavy composition of UHECRs at the highest energies measured by PAO, a plausible scenario for cosmic rays from the direction of Centaurus A can be found if they consist of intermediate-mass nuclei. The chemical composition of cosmic rays can be further constrained by lower-energy cosmic rays of the same rigidity. We find that cosmic ray acceleration in the lobes of Centaurus A is not favored, while acceleration in the stellar winds that are rich in intermediate-mass nuclei, could meet the requirement. This suggests that the observed excess may originate from cosmic ray accelerators induced by stellar explosions in the star-forming regions of Centaurus A and/or the Centaurus cluster located behind Centaurus A.