Physical Conditions and Particle Acceleration in the Kiloparsec Jet of Centaurus A

Abstract

The non-thermal emission from the kiloparsec-scale jet of Centaurus A exhibits two notable features, bright diffuse emission and many compact knots, which have been intensively studied in X-ray and radio observations. H.E.S.S. recently reported that the very-high-energy gamma-ray emission from this object is extended along the jet direction beyond a kiloparsec from the core. Here, we combine these observations to constrain the physical conditions of the kpc-jet and study the origin of the non-thermal emission. We show that the diffuse jet is weakly magnetized (${\eta }_B\sim 10^{-2}$) and energetically dominated by thermal particles. We also show that knots are the sites of both amplified magnetic field and particle (re-)acceleration. To keep sufficient energy in thermal particles, the magnetic and non-thermal particle energy in the knot regions are tightly constrained. The most plausible condition is an energy equipartition between them, ${\eta }_B\sim {\eta }_e\sim 0.1$. Such weak magnetic energy implies that particles in the knots are in the slow cooling regime. We suggest that the entire kpc-scale diffuse emission could be powered by particles that are accelerated at and escaped from knots.

Main text 6 pages, 2 figures, accepted for publication in ApJ Letters

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• Spatial variations of magnetic field along active galactic nuclei jets on sub-pc to Mpc scales
• The origin of the very-high-energy radiation along the jet of Centaurus A