Relativistic Corrections to the Sunyaev-zel’dovich Effect

We present an extension of the Kompaneets equation which allows relativistic effects to be included to any desired order. Using this, we are able to obtain simple analytic forms for the spectral changes due to the Sunyaev-Zel’dovich effect in hot clusters, correct to first and second order in the expansion parameter θe = kBTe/mc . These analytic forms agree with previous numerical calculations of the effect based upon the multiple scattering formalism, and are expected to be very accurate over all regions of the CMB spectrum for kBTe up to ∼ 10 keV. Our results confirm previous conclusions that the result of including relativistic corrections in the Sunyaev-Zel’dovich effect is a small reduction in the amplitude of the effect over the majority of the spectrum: specifically we find ∆T/T = −2y(1 − 17/10 θe + 123/40 θ e ) (correct to second-order) in the Rayleigh-Jeans region, where y is the usual Comptonization parameter. For a typical cluster temperature of 8 keV, this amounts to a correction downwards to the value of the Hubble constant derived using combined X-ray and Rayleigh-Jeans SunyaevZel’dovich information by about 5 percent. Subject headings: cosmology: cosmic microwave background — galaxies: clusters: general — radiative transfer — scattering