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compST: Comptonization, Sunyaev & Titarchuk

A Comptonization spectrum after Sunyaev & 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 $Nf/4\pi d^2$, where $N$ is the total number of photons from the source, $d$ is the distance to the source, f is the factor $f=z(z+3)y^z/\Gamma(2z+4)/\Gamma(z)$, $z$ is the spectral index, $y$ is the injected photon energy in units of the temperature, and $\Gamma$ is the incomplete gamma function.