Multiwavelength study of Cygnus A III. Evidence for relic lobe plasma
  We study the particle energy distribution in the cocoon surrounding Cygnus A,
using radio images between 151 MHz and 15 GHz and a 200 ks Chandra ACIS-I
image. We show that the excess low frequency emission in the the lobe further
from Earth cannot be explained by absorption or excess adiabatic expansion of
the lobe or a combination of both. We show that this excess emission is
consistent with emission from a relic counterlobe and a relic counterjet that
are being re-energized by compression from the current lobe. We detect hints of
a relic hotspot at the end of the relic X-ray jet in the more distant lobe. We
do not detect relic emission in the lobe nearer to Earth as expected from light
travel-time effects assuming intrinsic symmetry. We determine that the duration
of the previous jet activity phase was slightly less than that of the current
jet-active phase. Further, we explain some features observed at 5 and 15 GHz as
due to the presence of a relic jet.
