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

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.

💡 Research Summary

The paper investigates whether the nearby radio galaxy Centaurus A (Cen A) can be identified as a point source of ultra‑high‑energy cosmic rays (UHECR) detected by the Pierre Auger Observatory (PAO). The authors construct a “Cen A dominance model” in which a fraction fC of all UHECR with energies above 5.5 × 10¹⁹ eV originates from Cen A, while the remaining 1 − fC is assumed to be an isotropic background. To account for magnetic deflection in the intergalactic medium, the arrival directions of Cen A‑originated particles are smeared with a Gaussian of width θs (the smearing angle).

The statistical tool employed is the correlational angular distance distribution (CADD), which measures the distribution of angular separations between observed events and the model prediction. By scanning a grid of (fC, θs) values and comparing the simulated CADD with that derived from the 69 PAO events, the authors evaluate the goodness‑of‑fit using a Kolmogorov–Smirnov–type test, obtaining a maximum probability Pmax = 0.29 at fC ≈ 0.10 and θs ≈ 5°. In terms of the PAO exposure, this corresponds to an observed Cen A fraction fC,PAO ≈ 0.15, i.e., roughly ten of the 69 events.

Considering statistical uncertainties, the 2σ confidence region expands to 0.09 ≲ fC,PAO ≲ 0.25 and 0° ≲ θs ≲ 20°. The authors interpret the best‑fit smearing angle as an average magnetic deflection of about 5°, which, given the distance to Cen A (~3.8 Mpc) and the particle energies, implies an intergalactic magnetic field of order 10 nG in the vicinity of the source.

The study acknowledges several caveats. The isotropic background assumption neglects large‑scale structure anisotropies that could mimic a localized excess. The Gaussian smearing model is a simplification of the true turbulent magnetic field topology. Moreover, the limited sample size (69 events) yields relatively large statistical errors. Nonetheless, the analysis demonstrates a statistically significant excess of UHECR in the direction of Cen A, supporting the hypothesis that Cen A contributes a non‑negligible fraction of the observed ultra‑high‑energy cosmic‑ray flux. Future observations with larger event samples and more sophisticated magnetic‑field modeling will be essential to confirm and refine these conclusions.