# Non-thermal Sunyaev-Zel'dovich effect

Exactly what is the non-thermal Sunyaev-Zel'dovich effect? From what I understand from reading several papers by Mark Birkinshaw and Sergio Colafrancesco, I get the rough idea that the non-thermal SZ effect has something to do with it being in the relativistic realm.

Papers by Sergio Colafrancesco:

Review by Mark Birkinshaw:

The Sunyaev-Zel'dovich effect

To clarify, I'm not referring to the kinematic SZ effect.

There appears to be a distinction by the authors between the SZ effect due to hot thermal electrons and relativistic electrons. I find the nomenclature ambiguous, as temperature is essentially kinetic energy on a smaller scale, so why would relativistic electrons that have more kinetic energy be 'non-thermal'?

Lastly, is the key distinction between them using a relativistic vs. a non-relativistic formulation?

The rest-mass energy of an electron is 0.511 keV. In order to attain "relativistic energies" then the electrons must have kinetic energies similar to this or higher. The average kinetic energy of a particle due to its temperature is just $$3k_{B}T/2$$. If we equate this to 0.511 keV then the temperature required to have thermal relativistic electrons is $$T > 4 \times 10^{6}$$ K.