Relativistic Corrections to the Sunyaev-Zel'dovich Effect for Clusters of Galaxies

We study the generalized Kompaneets equation (kinetic equation for the photon distribution function taking into account the Compton scattering by electrons) using a relativistically covariant formalism. Using the generalized Kompaneets equation, we derive an analytic expression for the Sunyaev-Zel'dovich effect, which takes into account up to O(theta**5) terms, where theta=kT/mc**2 is the relativistic expansion parameter and T is the electron temperature. We carefully study the applicable region of the obtained analytic expression by comparing with the result of the direct numerical integration. We conclude that the present analytic expression can be reliably applied to the calculation of the Sunyaev-Zel'dovich effect for kT < 15keV, which is the highest electron temperature in the presently known clusters of galaxies. Therefore, the present analytic expression can be applied to all known clusters of galaxies.