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

TL;DR

This paper explores particle acceleration near Cen A's black hole, showing electrons can reach high energies producing TeV gamma rays, but protons likely require jet shear acceleration for ultra-high energy cosmic rays.

Contribution

It demonstrates that electron acceleration near the black hole can produce TeV gamma rays consistent with observations, and suggests jet shear acceleration as a mechanism for UHECR protons.

Findings

Electrons can reach Lorentz factors of 10^7-10^8 near the black hole.

Inverse Compton scattering explains observed TeV gamma-ray spectrum.

Protons are unlikely to be accelerated to UHECR energies near the black hole, but jet shear acceleration may achieve this.

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 $r_{\rm L}/c \sim 1$ hr 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.