Long Term Monitoring of the Dynamics and Particle Acceleration of Knots in the Jet of Centaurus A

TL;DR

This study analyzes 20 years of radio and X-ray data of Centaurus A's jet knots, revealing that steady shocks, not impulsive acceleration, likely produce the observed knots and their emissions, with some knots showing apparent motion.

Contribution

It provides the first long-term, multi-frequency observational analysis of jet knots in Centaurus A, challenging impulsive acceleration models and supporting steady shock formation mechanisms.

Findings

Steady shocks likely produce stationary knots with prolonged particle acceleration.

No evidence of impulsive acceleration or extreme variability in X-ray knots.

Some knots exhibit apparent motion, but no conclusive evidence for a faster jet spine.

Abstract

We present new and archival multi-frequency radio and X-ray data for Centaurus A obtained over almost 20 years at the VLA and with Chandra, with which we measure the X-ray and radio spectral indices of jet knots, flux density variations in the jet knots, polarization variations, and proper motions. We compare the observed properties with current knot formation models and particle acceleration mechanisms. We rule out impulsive particle acceleration as a formation mechanism for all of the knots as we detect the same population of knots in all of the observations and we find no evidence of extreme variability in the X-ray knots. We find the most likely mechanism for all the stationary knots is a collision resulting in a local shock followed by a steady state of prolonged, stable particle acceleration and X-ray synchrotron emission. In this scenario, the X-ray-only knots have radio counterparts that are too faint to be detected, while the radio-only knots are due to weak shocks where no particles are accelerated to X-ray emitting energies. Although the base knots are prime candidates for reconfinement shocks, the presence of a moving knot in this vicinity and the fact that there are two base knots are hard to explain in this model. We detect apparent motion in three knots; however, their velocities and locations provide no conclusive evidence for or against a faster moving `spine' within the jet. The radio-only knots, both stationary and moving, may be due to compression of the fluid.