The source of the cosmic-ray excess in the Centaurus region -- constraints on possible candidates, mass composition and cosmic magnetic fields

Abstract

The most significant excess in the arrival directions of ultra-high-energy cosmic rays with energies $\gtrsim40\,\mathrm{EeV}$ is found in the direction of several interesting source candidates, most prominently the nearby radio galaxy Centaurus A. Naturally, Cen A has been suspected to create the anisotropy - but very different scenarios have been proposed. This includes a subdominant source contribution in combination with isotropic background sources, as well as a scenario where Cen A supplies the whole cosmic-ray flux above the ankle. Recently, it was suggested that the overdensity could instead consist of strongly deflected events from the Sombrero galaxy. Thanks to the recent development of several models of the Galactic magnetic field, it is now possible to test these proposed scenarios explicitly. Leveraging the measured overdensity direction, significance, angular scale, and energy evolution, we place limits on the allowed signal fraction, the possible ejected charge number and the strength of the extragalactic magnetic field between the respective source and Earth. We find that the scenario of a subdominant source in the overdensity region requires the charge number to be $Z\lesssim6$ and the extragalactic magnetic field quantity $B/\mathrm{nG} \sqrt{L_c/\mathrm{Mpc}}$ to be between $~1$ and $~100$. For the Sombrero galaxy to be the source, the dominant charge number has to be around $Z=6$ with $1\lesssim B/\mathrm{nG} \sqrt{L_c/\mathrm{Mpc}}\lesssim20$. We find that a scenario where all the flux above $30\,\mathrm{EeV}$ is supplied by Cen A or M83 is possible for $20\lesssim B/\mathrm{nG} \sqrt{L_c/\mathrm{Mpc}}\lesssim30$ and a mixed composition - explaining both the Centaurus region excess and the distribution of the highest-energy events - however, another contributing source is possibly required in the energy range $<30\,\mathrm{EeV}$.