Multiwavelength study of Cygnus A – II. X-ray inverse-Compton emission from a relic counterjet and implications for jet duty cycles

The duty cycles of powerful radio galaxies and quasars such as the prototype Cygnus A are 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 farther lobes, separated by many tens of kiloparsecs and thus by tens of thousands of years in light-travel time, yield additional temporal resolution in studies of the life-cycles 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. The outer part of this counterjet does not overlie the current counterjet detected in radio emission, excluding the possibility that we are detecting the current counterjet in X-rays. This non-thermal X-ray emission has a power-law photon index of 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 make the following conclusions. (1) This emission is from a relic jet, indicating a previous episode of jet activity that took place prior to 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 constrain the time-scale between successive episodes of jet activity to ∼10 6 yr. (3) Transverse expansion of the jet causes expansion losses, which shift the energy distribution to lower energies. Particles with initially high Lorentz factors, which originally gave detectable synchrotron radiation, attain Lorentz factors ∼10 3 and scatter CMB photons, to give X-ray emission. (4) Assuming that the electrons cooled as a result of 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 , seems to emerge from the central nucleus of this active galaxy.