A Comptonization spectrum after
Sunyaev and Titarchuk 1980, A&A, 86, 121. This model is the
Comptonization of cool photons on hot electrons.

par1

temperature in keV

par2

optical depth

norm

,
where N is the total number of photons from the source, d is
the distance to the source, and f is the factor . z is the spectral
index, y is the injected photon energy in units of the temperature,
and G is the incomplete gamma
function.