# Variability and Proper Motion of X-ray Knots in the Jet of Centaurus A

## TL;DR

This study analyzes 15 years of Chandra X-ray observations of Centaurus A's jet, revealing proper motion, brightness variability, and insights into knot origins, with implications for jet physics.

## Contribution

It provides the first evidence of collective proper motion in X-ray knots and distinguishes between stationary and moving knots, advancing understanding of jet dynamics.

## Key findings

- Proper motion detected in faint X-ray knots ($0.68 ext{c}$)
- Brightness variations up to 27% observed in knots
- Stationary knots likely caused by jet-obstacle interactions

## Abstract

We report results from Chandra observations analyzed for evidence of variability and proper motion in the X-ray jet of Centaurus A. Using data spanning 15 years, collective proper motion of $11.3\pm3.3$ mas yr$^{-1}$, or $0.68\pm0.20c$, is detected for the fainter X-ray knots and other substructure present within the jet. The three brightest knots (AX1A, AX1C, and BX2) are found to be stationary to an upper limit of $0.10c$. Brightness variations up to 27% are detected for several X-ray knots in the jet. For the fading knots, BX2 and AX1C, the changes in spectral slope expected to accompany synchrotron cooling are not found, ruling it out and placing upper limits of $\simeq 80\rm\ \mu G$ for each of their magnetic field strengths. Adiabatic expansion can account for the observed decreases in brightness. Constraints on models for the origin of the knots are established. Jet plasma overrunning an obstacle is favored as the generator of stationary knots, while moving knots are likely produced either by internal differences in jet speed or the late stages of jet interaction with nebular or cloud material.