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

TL;DR

This paper investigates the physical conditions and particle acceleration mechanisms in the kiloparsec-scale jet of Centaurus A, combining multi-wavelength observations to constrain magnetic fields, thermal particle dominance, and energy distribution.

Contribution

It provides new constraints on the magnetic field strength, energy partition, and acceleration sites in the jet, highlighting the role of knots as key regions for particle acceleration and emission.

Findings

Diffuse jet is weakly magnetized with $ ext{η}_B extasciitilde 10^{-2}$

Knots are sites of magnetic field amplification and particle acceleration

Energy equipartition likely exists between magnetic and non-thermal particles in knots

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 ($\eta_B\sim10^{-2}$) 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, $\eta_B\sim\eta_e\sim0.1$. 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.