Superluminal proper motion in the X-ray jet of Centaurus A

TL;DR

This study analyzes Chandra X-ray observations of Centaurus A's jet, revealing superluminal motion in one knot and providing new constraints on jet inclination and velocity, indicating distinct X-ray and radio jet structures.

Contribution

Developed a systematic algorithm to measure proper motions in X-ray jet knots, discovering superluminal motion and highlighting differences between X-ray and radio jet features.

Findings

Detected superluminal proper motion of 2.7c in one X-ray jet knot.

Constrained jet inclination to less than 41 degrees.

Found X-ray and radio jets trace different structures.

Abstract

The structure of the jet in Cen A is likely better revealed in X-rays than in the radio band, which is usually used to investigate jet proper motions. In this paper, we analyze Chandra ACIS observations of Cen A from 2000 to 2022 and develop an algorithm for systematically fitting the proper motions of its X-ray jet knots. Most of the knots had an apparent proper motion below the detection limit. However, one knot at a transverse distance of $520~\mathrm{pc}$ had an apparent superluminal proper motion of $2.7\pm0.4~\mathrm{c}$. This constrains the inclination of the jet to be $i<41\pm6^{\circ}$, and the velocity of this knot to be $\beta>0.94\pm0.02$. This agrees well with the inclination measured in the inner jet by the EHT, but contradicts previous estimates based on jet and counterjet brightness. It also disagrees with the proper motion of the corresponding radio knot, of $0.8\pm0.1~\mathrm{c}$, which further indicates that the X-ray and radio bands trace distinct structures in the jet. There are four prominent X-ray jet knots closer to the nucleus, but only one of these is inconsistent with being stationary. A few jet knots also have a significant proper motion component in the non-radial direction. This component is typically larger closer to the center of the jet. We also detect brightness and morphology variations at a transverse distance of $100~\mathrm{pc}$ from the nucleus.