Hadronic-Origin TeV gamma-Rays and Ultra-High Energy Cosmic Rays from Centaurus A

Centaurus A (Cen A) is the nearest radio-loud AGN and is detected from radio to very high energy gamma-rays. Its nuclear spectral energy distribution (SED) shows a double-peak feature, which is well explained by the leptonic synchrotron + synchrotron self-Compton model. This model however cannot account for the observed high energy photons in the TeV range, which display a distinct component. Here we show that ~ TeV photons can be well interpreted as the neutral pion decay products from p-gamma interactions of Fermi accelerated high energy protons in the jet with the seed photons around the second SED peak at ~170 keV. Extrapolating the inferred proton spectrum to high energies, we find that this same model is consistent with the detection of 2 ultra-high-energy cosmic ray events detected by Pierre Auger Observatory from the direction of Cen A. We also estimate the GeV neutrino flux from the same process, and find that it is too faint to be detected by current high-energy neutrino detectors.

💡 Research Summary

The authors address a long‑standing problem in the broadband emission of the nearby radio‑loud active galaxy Centaurus A (Cen A). While the low‑energy (radio‑optical) and the first high‑energy peak (≈ 170 keV) of the nuclear spectral energy distribution (SED) are well reproduced by a standard one‑zone synchrotron plus synchrotron‑self‑Compton (SSC) model, the very‑high‑energy (VHE) γ‑ray component measured by HESS (0.1–10 TeV) cannot be accommodated by the same leptonic scenario. In particular, a dip around 10 GeV seen by Fermi‑LAT indicates an excess of TeV photons beyond the SSC extrapolation.

The paper proposes that this excess originates from hadronic processes: relativistic protons accelerated in the jet interact with the dense photon field associated with the SSC peak (≈ 170 keV) via the Δ‑resonance (p + γ → Δ⁺ → p π⁰ or n π⁺). The resonance condition Eₚ · ε_γ ≈ 0.32 GeV² (in the jet comoving frame) requires protons of ≈ 13 TeV (comoving) or ≈ 13 PeV (observer frame) when the target photons are at 170 keV. Each neutral pion carries ~20 % of the proton energy and decays into two γ‑rays, each with an energy of roughly 190 GeV in the observer frame, matching the HESS data.

Adopting the SSC parameters from previous fits (Doppler factor D = 1, bulk Lorentz factor Γ = 7, blob radius R′ ≈ 3 × 10¹⁵ cm, magnetic field B′ ≈ 6.2 G), the authors compute the comoving photon density n′γ ≈ 1.4 × 10⁶ cm⁻³ and the pγ optical depth τ{pγ} ≈ 2 × 10⁻⁶. Although τ_{pγ} is tiny, a sufficiently hard proton spectrum can still generate the observed TeV flux. The proton distribution is modeled as a broken power law: dN/dE ∝ E^{-α} with α = 2 below a break energy E_{b,p} (to keep the total proton power below the Eddington limit) and α = 3.08 above it. By varying E_{b,p} between 4 PeV and 25 PeV the resulting π⁰‑induced γ‑ray component reproduces the HESS points for a wide range of break energies.

The same proton population can be extrapolated to ultra‑high energies. Using the Hillas‑type limit E_{p,max} ≈ 4 × 10¹⁹ eV · (B′/6.2 G)·(t_{var}/10⁵ s)·Γ, the authors argue that protons could reach at least 57 EeV, the energy of the two Pierre Auger Observatory (PAO) events that lie within 3° of Cen A. Relating the observed γ‑ray flux at 190 GeV to the proton flux via the pγ interaction efficiency (F_p ≈ 7.5 F_γ/τ_{pγ}), they estimate a proton flux at 57 EeV of ≈ 1.6 × 10⁻¹³ erg cm⁻² s⁻¹. Accounting for PAO’s exposure (≈ 9000/π km² sr), the declination correction (ω ≈ 0.64) and a 3.75‑year data‑taking period, the expected number of detected UHECRs is N ≈ 3.7 ζ, where ζ is the fraction of protons that escape the source. For a plausible ζ ≈ 0.5, the prediction (≈ 1.9 events) matches the two observed events, supporting the hadronic scenario.

Neutrino production follows from the charged‑pion channel (π⁺ → μ⁺ ν_μ → e⁺ ν_e ν̄_μ ν_μ). Since the Δ‑resonance yields π⁰ and π⁺ in a 2:1 ratio, the total neutrino energy flux is F_ν ≈ (3/8) F_γ. This translates to a maximal neutrino flux of ≈ 2.5 × 10⁻¹³ erg cm⁻² s⁻¹ (≈ 1.5 × 10⁻¹⁰ GeV cm⁻² s⁻¹) at ≈ 95 GeV, well below the current IceCube upper limits, explaining the non‑detection of a neutrino counterpart.

The discussion emphasizes that the model requires a proton‑to‑electron luminosity ratio of 10³–10⁴, with the total proton power approaching but not exceeding the black‑hole Eddington luminosity (L_Edd ≈ 1.3 × 10⁴⁶ erg s⁻¹). The authors acknowledge uncertainties in the Doppler factor, photon density, and jet composition, but argue that reasonable parameter choices can simultaneously satisfy the TeV γ‑ray spectrum, the Fermi‑LAT dip, and the PAO UHECR events. They also note an alternative explanation via electromagnetic cascades initiated by UHECR pγ interactions, but focus on the simpler Δ‑resonance picture.

In summary, the paper presents a coherent hadronic framework that links Cen A’s TeV γ‑ray excess, its SSC peak, and the observed ultra‑high‑energy cosmic‑ray events. The model predicts a faint GeV–TeV neutrino flux, consistent with current non‑detections, and highlights Cen A as a promising laboratory for studying particle acceleration and hadronic processes in radio‑loud AGN jets. Future deeper TeV observations and next‑generation neutrino telescopes will be crucial to test the proposed scenario.