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 _ { L } / c \sim 1 hr for a typical light cylinder radius of r _ { L } \simeq 5 r _ { 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 .

Conclusions :