A Chandra Study of the Lobe/ISM Interactions Around the Inner Radio Lobes of Centaurus A: Constraints on the Temperature Structure and Transport Processes

TL;DR

This study uses deep Chandra X-ray observations to analyze the shock interactions between the radio lobe and the interstellar medium in Centaurus A, revealing temperature structures and transport processes that influence the energy estimates of the outburst.

Contribution

It provides the first detailed analysis of the temperature discontinuity and shock properties around Centaurus A's radio lobe, highlighting non-equilibrium conditions between electrons and protons.

Findings

The shock is a strong overpressure feature with a sharp surface brightness discontinuity.

Electron and proton temperatures do not equilibrate quickly, affecting energy estimates.

The proton temperature may be significantly higher than the electron temperature, implying larger outburst energy.

Abstract

We present results from deeper {\em Chandra} observations of the southwest radio lobe of Centaurus A, first described by Kraft et al. (2003). We find that the sharp X-ray surface brightness discontinuity extends around $\sim$75% of the periphery of the radio lobe, and detect significant temperature jumps in the brightest regions of this discontinuity nearest to the nucleus. This demonstrates that this discontinuity is indeed a strong shock which is the result of an overpressure which has built up in the entire lobe over time. Additionally, we demonstrate that if the mean free path for ions to transfer energy and momentum to the electrons behind the shock is as large as the Spitzer value, the electron and proton temperatures will not have equilibrated along the SW boundary of the radio lobe where the shock is strongest. Thus the proton temperature of the shocked gas could be considerably larger than the observed electron temperature, and the total energy of the outburst correspondingly larger as well. We investigate this using a simple one-dimensional shock model for a two-fluid (proton/electron) plasma. We find that for the thermodynamic parameters of the Cen A shock the electron temperature rises rapidly from $\sim$0.29 keV (the temperature of the ambient ISM) to $\sim$3.5 keV at which point heating from the protons is balanced by adiabatic losses. The proton and electron temperatures do not equilibrate in a timescale less than the age of the lobe. We note that the measured electron temperature of similar features in other nearby powerful radio galaxies in poor environments may considerably underestimate the strength and velocity of the shock.