Refined approximations for the distortion visibility function and μ-type spectral distortions

The physical processes affecting the thermalization of cosmic microwave background spectral distortions are very simple and well understood. This allows us to make precise predictions for the distortions signals caused by various energy release scenarios, where the theoretical uncertainty is largely dominated by the physical ingredients that are used for the calculation. Here, we revisit various approximations for the distortion visibility function - defined using the fraction of the released energy that does not thermalize - and early mu-type distortions. Our approach is based on a perturbative expansion, which allows us to identify and clarify the origin of different improvements over earlier approximations. It provides a better than similar or equal to 0.1-1 per cent description of our numerical results over a wide range of parameters. In particular, we are able to capture the high-frequency part of the mu-distortion, which directly depends on the time derivative of the electron temperature. We also include lowest order double Compton and Compton scattering relativistic corrections, finding that because of cancellation they increase the thermalization efficiency in the tail of the distortion visibility function by only similar or equal to 10 per cent (at z similar or equal to 6 x 10(6)), although individually their effect can reach similar or equal to 20-40 per cent.