The Cocoon Shocks of Cygnus A: Pressures and Their Implications for the Jets and Lobes

TL;DR

This study uses extensive Chandra X-ray observations to analyze the cocoon shocks of Cygnus A, revealing detailed shock properties, jet dynamics, and pressure distributions, which inform models of jet composition and energy transport.

Contribution

It provides the first detailed measurements of cocoon shock Mach numbers, pressures, and jet properties in Cygnus A, supporting models of light, internally energized jets with mildly relativistic speeds.

Findings

Shock Mach numbers range from 1.18 to 1.66.

Total outburst energy estimated at approximately 4.7×10^{60} erg.

Jets are likely light, carrying energy mainly through internal plasma energy.

Abstract

We use 2.0 Msec of Chandra observations to investigate the cocoon shocks of Cygnus A and some implications for its lobes and jet. Measured shock Mach numbers vary in the range 1.18-1.66 around the cocoon. We estimate a total outburst energy of $\simeq 4.7\times10^{60}\rm\ erg$, with an age of $\simeq 2 \times 10^{7}\rm\ yr$. The average postshock pressure is found to be $8.6 \pm 0.3 \times 10^{-10}\rm\ erg\ cm^{-3}$, which agrees with the average pressure of the thin rim of compressed gas between the radio lobes and shocks, as determined from X-ray spectra. However, average rim pressures are found to be lower in the western lobe than in the eastern lobe by $\simeq 20\%$. Pressure estimates for hotspots A and D from synchrotron self-Compton models imply that each jet exerts a ram pressure $\gtrsim$ 3 times its static pressure, consistent with the positions of the hotspots moving about on the cocoon shock over time. A steady, one-dimensional flow model is used to estimate jet properties, finding mildly relativistic flow speeds within the allowed parameter range. Models in which the jet carries a negligible flux of rest mass are consistent with with the observed properties of the jets and hotspots. This favors the jets being light, implying that the kinetic power and momentum flux are carried primarily by the internal energy of the jet plasma rather than by its rest mass.