Dynamics Inside the Radio and X-ray Cluster Cavities of Cygnus A and Similar FRII Sources

TL;DR

This study models the dynamical evolution of material inside radio lobes and X-ray cavities in FRII sources like Cygnus A, explaining observed features through axisymmetric simulations of plasma, cosmic rays, and magnetic fields.

Contribution

It presents a novel axisymmetric simulation approach to understand the evolution of plasma, cosmic rays, and magnetic fields in radio lobes and cavities of FRII sources, matching several observations.

Findings

Boundary backflow explains observed features.

Computed radio lobe emission matches limb-brightening observations.

Offsets between X-ray and radio emissions are naturally produced by winds.

Abstract

We describe approximate axisymmetric computations of the dynamical evolution of material inside radio lobes and X-ray cluster gas cavities in Fanaroff-Riley II sources such as Cygnus A. All energy is delivered by a jet to the lobe/cavity via a moving hotspot where jet energy dissipates in a reverse shock. Our calculations describe the evolution of hot plasma, cosmic rays (CRs) and toroidal magnetic fields flowing from the hotspot into the cavity. Many observed features are explained. Gas, CRs and field flow back along the cavity surface in a "boundary backflow" consistent with detailed FRII observations. Computed ages of backflowing CRs are consistent with observed radio-synchrotron age variations only if shear instabilities in the boundary backflow are damped and we assume this is done with viscosity of unknown origin. Magnetic fields estimated from synchrotron self-Compton (SSC) X-radiation observed near the hotspot evolve into radio lobe fields. Computed profiles of radio synchrotron lobe emission perpendicular to the jet are dramatically limb-brightened in excellent agreement with FRII observations although computed lobe fields exceed those observed. Strong winds flowing from hotspots naturally create kpc-sized spatial offsets between hotspot inverse Compton (IC-CMB) X-ray emission and radio synchrotron emission that peaks 1-2 kpc ahead where the field increases due to wind compression. In our computed version of Cygnus A, nonthermal X-ray emission increases from the hotspot (some IC-CMB, mostly SSC) toward the offset radio synchrotron peak (mostly SSC). A faint thermal jet along the symmetry axis may be responsible for redirecting the Cygnus A non-thermal jet.