The duty-cycle of powerful radio galaxies and quasars such as the prototype Cygnus A is poorly understood .
X-ray observations of inverse-Compton scattered Cosmic Microwave Background ( ICCMB ) photons probe lower Lorentz-factor particles than radio observations of synchrotron emission and thus potentially reveal a more aged population .
Comparative studies of the nearer and further lobes , separated by many 10s of kpc and thus 10s of thousands of years in light-travel time , yield additional temporal resolution in studies of the lifecycles of such objects .
We have co-added all archival Chandra ACIS-I data and present a deep 200 ks image of Cygnus A .
This deep image reveals the presence of X-ray emission from a counterjet i.e .
a jet receding from Earth and related to a previous episode of jet activity .
The outer part of this counterjet does not overlay the current counterjet detected in radio emission , excluding the possibility that we detect the current counterjet in X-rays .
The non-thermal X-ray emission has a power-law photon index is 1.7 , and we interpret this emission as ICCMB radiation .
There is an absence of any discernible X-ray emission associated with a jet flowing towards Earth .
We conclude that : ( 1 ) The emission from a relic jet , indicates a previous episode of jet activity , that took place earlier than the current jet activity appearing as synchrotron radio emission .
( 2 ) The presence of X-ray emission from a relic counterjet of Cygnus A and the absence of X-ray emission associated with any relic approaching jet constrains the timescale between successive episodes of jet activity to \sim 10 ^ { 6 } years .
( 3 ) Transverse expansion of the jet causes expansion losses which shifts the energy distribution to lower energies .
Particles with initially high Lorentz factors , that originally gave detectable synchrotron radiation , attain Lorentz factors \sim 10 ^ { 3 } and inverse-Compton scatter CMB photons , to give X-ray emission .
( 4 ) Assuming the electrons cooled due to adiabatic expansion , the required magnetic field strength is substantially smaller than the equipartition magnetic field strength .
( 5 ) A high minimum Lorentz factor for the distribution of relativistic particles in the current jet , of a few 10 ^ { 3 } , is ejected from the central nucleus of this active galaxy .