Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-12-19
Mon.Not.Roy.Astron.Soc. 349 (2004) 336
Astronomy and Astrophysics
Astrophysics
12 pages, 9 figures, accepted to MNRAS
Scientific paper
10.1111/j.1365-2966.2004.07511.x
We explore the energetics and plasma composition in FR II sources using a new simple method of combining shock dynamics and radiation spectrum. The hot spots are identified with the reverse shocked region of jets. With the one-dimensional shock jump conditions taking account of the finite pressure of hot ICM, we estimate the rest mass and energy densities of the sum of thermal and non-thermal particles in hot spots. Independently, based on the Synchrotron Self-Compton (SSC) model, we estimate the number and energy densities of {\it non-thermal} electrons using the multi-frequency radiation spectrum of hot spots. 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 3C123, we find that the energy density of non-thermal electrons is about 10 times larger than that of magnetic field. We find that the 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 since the number density of non-thermal electrons exceeds that of the total number density. Hence, we find that only pure $e^{\pm}$ 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.
Kino Motoki
Takahara Fumio
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