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

The nonthermal 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 kiloparsec jet and study the origin of the nonthermal emission. We show that the diffuse jet is weakly magnetized (${\eta }_{{\rm{B}}}\sim {10}^{-2}$) 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 nonthermal particle energy in the knot regions is tightly constrained. The most plausible condition is an energy equipartition between them, ${\eta }_{{\rm{B}}}\sim {\eta }_{e}\sim 0.1$. Such weak magnetic energy implies that particles in the knots are in the slow cooling regime. We suggest that the entire kiloparsec-scale diffuse emission could be powered by particles that are accelerated at and escaped from knots.