2009A&A...506L..41R

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. 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. We analyze the constraints on the achievable particle energies imposed by radiative losses and corotation for conditions inferred from observations. We show that for an underluminous source such as Cen A, centrifugally accelerated electrons may reach Lorentz factors of up to γ~(10⁷-10⁸), 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 10³⁹ erg/s. Upscattering of Comptonized disk photons is expected to lead to a TeV spectrum L_ν∝ν^–αc_^ with a spectral index α_c≃(1.5-1.9), consistent with HESS results. The corresponding minimum variability timescale could be as low as r_L/c∼1h for a typical light cylinder radius of r_L≃5r_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 50EeV.