Centaurus A as a point source of Ultra-High Energy Cosmic Rays

Abstract

We probe the possibility that Centaurus A (Cen A) is a point source of ultra-high energy cosmic rays (UHECR) observed by PAO, through the statistical analysis of the arrival direction distribution. For this purpose, we set up the Cen A dominance model for the UHECR sources, in which Cen A contributes the fraction $f_{\rm C}$ of the whole UHECR with energy above $5.5\times10^{19}\,{\rm eV}$ and the isotropic background contributes the remaining $1-f_{\rm C}$ fraction. The effect of the intergalactic magnetic fields on the bending of the trajectory of Cen A originated UHECR is parameterized by the gaussian smearing angle $\theta_s$. Using the correlational angular distance distribution (CADD), we identify the excess of UHECR in the Cen A direction and fit the CADD of the observed PAO data by varying two parameters $f_{\rm C}$ and $\theta_s$ of the Cen A dominance model. The best-fit parameter values are $f_{\rm C}\approx0.1$ (The corresponding Cen A fraction observed at PAO is $f_{\rm C,PAO}\approx0.15$, that is, about 10 out of 69 UHECR.) and $\theta_s=5^\circ$ with the maximum probability $P_{\rm max}=0.29$. Considering the uncertainty concerning the assumption of isotropic background in the Cen A dominance model, we extend the viable parameter ranges to the $2\sigma$ band, $0.09\lesssim f_{\rm C,PAO}\lesssim 0.25$ and $0^\circ\lesssim \theta_s\lesssim 20^\circ$. This result supports the existence of a point source extended by the intergalactic magnetic fields in the direction of Cen A. If Cen A is actually the source responsible for the observed excess of UHECR, the average deflection angle of the excess UHECR implies the order of $10\,{\rm nG}$ intergalactic magnetic field in the vicinity of Cen A.