Discovery of an X-ray cavity near the radio lobes of Cygnus A indicating
  previous AGN activity
  Cygnus A harbours the nearest powerful radio jet of an Fanaroff-Riley (FR)
class II radio galaxy in a galaxy cluster where the interaction of the jet with
the intracluster medium (ICM) can be studied in detail. We use a large set of
Chandra archival data, VLA and new LOFAR observations to shed new light on the
interaction of the jets with the ICM. We identify an X-ray cavity in the
distribution of the X-ray emitting plasma in the region south of the Cyg A
nucleus which has lower pressure than the surrounding medium. The LOFAR and VLA
radio observations show that the cavity is filled with synchrotron emitting
plasma. The spectral age and the buoyancy time of the cavity indicates an age
at least as large as the current Cyg A jets and not much larger than twice this
time. We suggest that this cavity was created in a previous active phase of Cyg
A when the energy output of the Active Galactic Nucleus (AGN) was about two
orders of magnitude less than today.
