Cen A as TeV gamma-ray and possible UHE cosmic-ray source

Abstract

Context: The most nearby active galaxy Cen A has attracted considerable attention as a detected TeV gamma-ray and possible ultra-high energy (UHE) cosmic-ray emitter. Aims: We investigate the efficiency of particle acceleration close to the supermassive black hole (BH) horizon assuming that accretion in the innermost part of the disk occurs in an advection-dominated (ADAF) mode. Methods: We analyze the constraints on the achievable particle energies imposed by radiative losses and corotation for conditions inferred from observations. Results: We show that for an underluminous source such as Cen A, centrifugally accelerated electrons may reach Lorentz factors of up to $\gamma \sim (10^7-10^8)$, allowing inverse Compton (Thomson) upscattering of ADAF sub-mm disk photons into the TeV regime with an associated maximum (isotropic) luminosity of the order of a few times $10^{39}$ erg/s. Upscattering of Comptonized disk photons is expected to lead to a TeV spectrum $L_{\nu }\propto {\nu }^{-{\alpha }_c}$ with a spectral index ${\alpha }_c\simeq (1.5-1.9)$, consistent with H.E.S.S. results. The corresponding minimum variability timescale could be as low as $r_{\mathrm{L}}/c\sim 1$ hr for a typical light cylinder radius of $r_{\mathrm{L}}\simeq 5r_{\mathrm{s}}$. While efficient electron acceleration appears to be well possible, protons are unlikely to be accelerated into the extreme UHECR regime close to the central black hole. We argue that if Cen A is indeed an extreme UHECR emitting source, then shear acceleration along the kpc-scale jet could represent one of the most promising mechanisms capable of pushing protons up to energies beyond 50 EeV.

4 pages, A&A Letters, in press

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• Fermi acceleration in astrophysical jets
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• Ultra High Energy Cosmic Rays from Black Hole Jets of Radio Galaxies
• Resolving the innermost parsec of Centaurus A at mid-infrared wavelengths
• Three-Dimensional Relativistic MHD Simulations of the Kelvin-Helmholtz Instability: Magnetic Field Amplification by a Turbulent Dynamo
• Chandra and XMM-Newton Observations of the Nucleus of Centaurus A
• New results on particle acceleration in the Centaurus A jet and counterjet from a deep Chandra observation
• Variable VHE gamma-ray emission from non-blazar AGNs
• Gemini Imaging of Mid-IR Emission from the Nuclear Region of Centaurus A
• On the efficiency of particle acceleration by rotating magnetospheres in AGN
• Transverse stability of relativistic two-component jets
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Cited by in corpus (35)

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• Implications of Strong Intergalactic Magnetic Fields for Ultra-High-Energy Cosmic-Ray Astronomy
• Evidence for a second component in the high-energy core emission from Centaurus A?
• The $\gamma$-ray spectrum of the core of Centaurus A as observed with H.E.S.S. and Fermi-LAT
• NuSTAR and XMM-Newton Observations of the Hard X-Ray Spectrum of Centaurus A
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• Non-thermal Processes in Black-Hole-Jet Magnetospheres
• One-zone SSC model for the core emission of Centaurus A revisited
• Galactic magnetic deflections and Centaurus A as a UHECR source
• LUNASKA Experiment Observational Limits on UHE Neutrinos from Centaurus A and the Galactic Center
• Magnetic Deflections of Ultra-High Energy Cosmic Rays from Centaurus A
• The Centaurus A Ultrahigh-Energy Cosmic Ray Excess and the Local Extragalactic Magnetic Field
• Constraining the Emissivity of Ultrahigh Energy Cosmic Rays in the Distant Universe with the Diffuse Gamma-ray Emission
• Particle Diffusion and Acceleration in Magnetorotational Instability Turbulence
• Neutrinos from stochastic acceleration in black hole environments
• A correlation between the highest energy cosmic rays and nearby active galactic nuclei detected by Fermi
• Synchrotron pair halo and echo emission from blazars in the cosmic web: application to extreme TeV blazars
• On the excess of ultra-high energy cosmic rays in the direction of Centaurus A
• Radio Galaxies with the Cherenkov Telescope Array
• Gamma-Rays from Non-Blazar AGN
• UHECR Echoes from the Council of Giants
• Very high-energy gamma-ray signature of ultrahigh-energy cosmic-ray acceleration in Centaurus A
• Time dependent gamma-ray production in the anisotropic IC $e^{\pm }$ pair cascade initiated by electrons in active galaxies
• Interpreting correlated observations of cosmic rays and gamma-rays from Centaurus A with a proton blazar inspired model
• Central accumulation of magnetic flux in massive Seyfert galaxies as a possible engine to trigger ultrahigh energy cosmic rays
• Search for ultra high energy cosmic ray sources. FRI radio galaxy Centaurus A
• Jets on all scales
• High-energy emission from jet-cloud interactions in AGNs
• Imaging and Spectral Fitting of Bright Gamma-ray Sources with the COSI Balloon Payload
• The H.E.S.S. Extragalactic Sky
• Explaining the UHECR spectrum, composition and large-scale anisotropies with radio galaxies