AChandraStudy of the Lobe/Interstellar Medium Interactions around the Inner Radio Lobes of Centaurus A: Constraints on the Temperature Structure and Transport Processes

Abstract

We present results from deeper Chandra observations of the southwest radio lobe of Centaurus A, first described by Kraft and coworkers. We find that the sharp X-ray surface brightness discontinuity extends around ~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 that is the result of an overpressure that has built up in the entire lobe over time. In addition, 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 southwest 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 ~0.29 keV (the temperature of the ambient ISM) to ~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.