Astronomy & Astrophysics

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Issue		A&A Volume 506, Number 3, November II 2009
Page(s)		L41 - L44
Section		Letters
DOI		10.1051/0004-6361/200912562
Published online		01 October 2009

A&A 506, L41-L44 (2009)
DOI: 10.1051/0004-6361/200912562

Letter

Centaurus A as TeV $\gamma$ -ray and possible UHE cosmic-ray source

F. M. Rieger^{1, 2} and F. A. Aharonian^{1, 3}

¹ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
e-mail: frank.rieger@mpi-hd.mpg.de
² European Associated Laboratory for Gamma-Ray Astronomy, jointly supported by CNRS and MPG, Europe
³ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland

Received 25 May 2009 / Accepted 18 September 2009

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 10³⁹ erg/s. Upscattering of Comptonized disk photons is expected to lead to a TeV spectrum $L_{\nu} \propto \nu^{-\alpha_{\rm c}}$ with a spectral index $\alpha_{\rm c} \simeq (1.5{-}1.9)$ , consistent with HESS results. The corresponding minimum variability timescale could be as low as $r_{\rm L}/c \sim 1$ h for a typical light cylinder radius of $r_{\rm L} \simeq 5\, r_{\rm s}$ . 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.

Key words: acceleration of particles -- black hole physics -- radiation mechanisms: non-thermal -- galaxies: active -- galaxies: jets

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Letter

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

Centaurus A as TeV $\gamma$ -ray and possible UHE cosmic-ray source