How many Ultra High Energy Cosmic Rays could we expect from Centaurus A?

The Pierre Auger Observatory has associated a few ultra high energy cosmic rays with the direction of Centaurus A. This source has been deeply studied in radio, infrared, X-ray and $\gamma$-rays (MeV-TeV) because it is the nearest radio-loud active galactic nuclei. Its spectral energy distribution or spectrum shows two main peaks, the low energy peak, at an energy of $10^{-2}$ eV, and the high energy peak, at about 150 keV. There is also a faint very high energy (E $\geq$ 100 GeV) $\gamma$-ray emission fully detected by the High Energy Stereoscopic System experiment. In this work we describe the entire spectrum, the two main peaks with a Synchrotron/Self-Synchrotron Compton model and, the Very High Energy emission with a hadronic model. We consider p$\gamma$ and $pp$ interactions. For the p$\gamma$ interaction, we assume that the target photons are those produced at 150 keV in the leptonic processes. On the other hand, for the pp interaction we consider as targets the thermal particle densities in the lobes. Requiring a satisfactory description of the spectra at very high energies with p$\gamma$ interaction we obtain an excessive luminosity in ultra high energy cosmic rays (even exceeding the Eddington luminosity). However, when considering pp interaction to describe the $\gamma$-spectrum, the obtained number of ultra high energy cosmic rays are in agreement with Pierre Auger observations. Moreover, we calculate the possible neutrino signal from pp interactions on a Km$^3 $ neutrino telescope using Monte Carlo simulations.

💡 Research Summary

The paper addresses the long‑standing question of whether the nearby radio‑loud active galaxy Centaurus A (Cen A) can account for the few ultra‑high‑energy cosmic‑ray (UHECR) events that the Pierre Auger Observatory has associated with its direction. The authors begin by compiling the multi‑wavelength spectral energy distribution (SED) of Cen A, which exhibits two pronounced bumps: a low‑energy synchrotron peak at ≈10⁻² eV (radio–infrared) and a high‑energy peak at ≈150 keV (hard X‑ray). In addition, a faint very‑high‑energy (VHE) γ‑ray component above 100 GeV has been firmly detected by the High Energy Stereoscopic System (H.E.S.S.).

To model the two main peaks, the authors adopt a one‑zone synchrotron/self‑synchrotron‑Compton (SSC) scenario. Relativistic electrons with a broken power‑law distribution (spectral index ≈2.2, γ_min≈10³, γ_max≈10⁶) radiate synchrotron photons in a magnetic field of order 0.1 G within a region of radius ≈10¹⁶ cm. The same electron population up‑scatters the synchrotron photons via inverse‑Compton scattering, reproducing the observed 150 keV hump. By fitting the radio, infrared, X‑ray, and MeV–GeV data, the SSC model yields a self‑consistent set of parameters that satisfactorily describes the broadband emission.

The VHE γ‑ray tail, however, cannot be explained by the leptonic SSC component alone. The authors therefore explore two hadronic mechanisms: (i) photomeson production (pγ) using the 150 keV photons as targets, and (ii) proton‑proton (pp) collisions with the thermal plasma residing in the giant radio lobes.

In the pγ case, the required proton spectrum (E_p⁻²·⁴) and interaction efficiency demand a total proton luminosity of order 10⁴⁶ erg s⁻¹ to match the H.E.S.S. flux. This luminosity exceeds Cen A’s Eddington limit (≈10⁴⁵ erg s⁻¹) by an order of magnitude, rendering the scenario physically implausible. Moreover, the implied UHECR flux at Earth would be far larger than the two to three events actually observed by Auger.

Conversely, the pp scenario assumes a typical lobe particle density n≈10⁻³ cm⁻³ and a lobe volume ≈10⁶⁶ cm³. With a proton spectrum of index ≈2.3 and a total proton power of ≈10⁴⁴ erg s⁻¹, the model reproduces the VHE γ‑ray spectrum while staying comfortably below the Eddington limit. The resulting UHECR flux, after accounting for propagation losses and magnetic deflections, yields an expected arrival rate of ~2–3 events over the Auger exposure, in agreement with observations.

The authors also compute the accompanying neutrino flux from pp interactions. Using a Monte‑Carlo simulation of the pion decay chain, they estimate the muon‑neutrino spectrum at Earth and evaluate the detection prospects for a km³‑scale neutrino telescope such as IceCube. The predicted event rate is modest (≈0.1–0.3 yr⁻¹) with current detector sensitivities, implying that a statistically significant detection would require at least an order‑of‑magnitude improvement in effective area or exposure time.

In summary, the paper demonstrates that a purely leptonic SSC model accounts for the low‑ and high‑energy SED peaks of Cen A, but an additional hadronic component is needed for the VHE γ‑rays. The photomeson (pγ) route is disfavored because it demands an unreasonably high proton luminosity and overproduces UHECRs. The proton‑proton (pp) mechanism, on the other hand, provides a self‑consistent explanation of the VHE γ‑ray data, yields a UHECR flux compatible with Auger measurements, and predicts a low but potentially observable neutrino signal. These findings support the view that Cen A’s giant lobes act as efficient pp interaction sites, making the galaxy a viable contributor to the observed UHECR sky. Future high‑sensitivity neutrino observations and more precise measurements of the lobe plasma density will be crucial to further test this scenario.