Cen A as gamma- and UHE cosmic-ray Source

TL;DR

This paper reviews Cen A as a source of gamma rays and ultra-high energy cosmic rays, discussing observed emissions and the challenges of acceleration mechanisms reaching ultra-high energies.

Contribution

It proposes a scenario for VHE emission origin near the black hole and analyzes acceleration mechanisms, highlighting their limitations in reaching ultra-high energies.

Findings

Cen A detected in HE and VHE gamma-ray domains.

Most acceleration mechanisms struggle to reach >10^{19} eV for protons.

Shear acceleration may be more effective than others.

Abstract

Cen A has been recently detected in the high-energy (HE) and very high energy (VHE) gamma-ray domain by Fermi-LAT and H.E.S.S. We review the observed characteristics and suggest a scenario where the VHE emission originates from the vicinity of the black hole. Motivated by the possible association of some ultra-high energy (UHE) cosmic ray (CR) events with Cen A, we further analyze the acceleration efficiency for a number of a mechanisms (including shock and stochastic acceleration), showing that most of them (apart perhaps from shear) have serious difficulties in accelerating protons beyond a few $10^{19}$ eV.