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

2009 
Context. The most nearby active galaxy Cen A has attracted considerable attention as a detected TeV gamma-ray and possible ultrahigh energy (UHE) cosmic-ray emitter. Aims. We investigate the effi ciency 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 toγ∼ (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 Lν∝ν −αc with a spectral indexαc≃ (1.5−1.9), consistent with H.E.S.S. results. The corresponding minimum variability timescale could be as low as rL/c∼ 1 hr for a typical light cylinder radius of rL≃ 5 rs. While effi cient 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.
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