Constraints on the energetics and plasma composition of relativistic jets in FR II sources

We explore the energetics and plasma composition in FR II sources using a new simple method of combining shock dynamics and radiation spectrum. The hotspots are identified with the reverse-shocked region of jets. With the 1D shock jump conditions taking account of the finite pressure of hot intracluster medium (ICM), we estimate the rest mass and energy densities of the sum of thermal and non-thermal particles in hotspots. Independently, based on the Synchrotron Self-Compton (SSC) model, we estimate the number and energy densities of non-thermal electrons using the multifrequency radiation spectrum of hotspots. We impose the condition that the obtained rest mass, internal energy and number densities of non-thermal electrons should be lower than those of the total particles determined by shock dynamics. We apply this method to Cygnus A. We examine three extreme cases of pure electron-positron pair plasma (Case I), pure electron-proton plasma with separate thermalization (Case II) and pure electron-proton plasma in thermal equilibrium (Case III). By detailed SSC analysis for Cygnus A and 3C 123, we find that the energy density of non-thermal electrons is about 10 times larger than that of the magnetic field. We find that Case III is not acceptable because predicted photon spectra do not give a good fit to the observed one. We find that Case II can also be ruled out because the number density of non-thermal electrons exceeds that of the total number density. Hence we find that only pure e ± plasma (Case I) is acceptable among the three cases. Total kinetic power of jet and electron acceleration efficiency are also constrained by internal energy densities of non-thermal and total particles.