The X-ray Ribs Within the Cocoon Shock of Cygnus A
  We use new and archival Chandra observations of Cygnus A, totalling $\sim$1.9
Ms, to investigate the distribution and temperature structure of gas lying
within the projected extent of the cocoon shock and exhibiting a rib-like
structure. We confirm that the X-rays are dominated by thermal emission with an
average temperature of around 4 keV, and have discovered an asymmetry in the
temperature gradient, with the southwestern part of the gas cooler than the
rest by up to 2 keV. Pressure estimates suggest that the gas is a coherent
structure of single origin located inside the cocoon, with a mass of roughly
$2\times10^{10} M_{\odot}$. We conclude that the gas is debris resulting from
disintegration of the cool core of the Cygnus A cluster after the passage of
the jet during the early stages of the current epoch of activity. The 4 keV gas
now lies on the central inside surface of the hotter cocoon rim. The
temperature gradient could result from an offset between the centre of the
cluster core and the Cygnus A host galaxy at the switch-on of current radio
activity.
