Discovery of an X-ray cavity near the radio lobes of Cygnus A indicating previous AGN activity

Abstract

Cygnus A harbours the nearest powerful radio jet of a 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 other new radio 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 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 that is at least as old as the current Cyg A jets and not much older 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 was about two orders of magnitude less than today.