simpl: comptonization of a seed spectrum

The SIMple Power Law model: An empirical model of Comptonization in which a fraction of the photons in an input seed spectrum is scattered into a power-law component (Steiner et al. 2009). It is designed for use with soft thermal spectra that are not Compton thick and that have a photon index Gamma >1. simpl offers the advantage of operating in a self consistent manner, linking the seed spectrum to the generated power law. Compared to powerlaw, simpl gives equally good fits while also employing just two parameters, and simpl has the virtue of eliminating the divergence of powerlaw at low energies. Because simpl redistributes input photons to higher (and lower energies), for detectors with limited response matrices (at high or low energies), or with poor resolution, the sampled energies should be extended to adequately cover the relevant energy range (for details and an example, see the appendix in Steiner et al. (2009)).

par1 Gamma, the photon power law index.
par2 The scattered fraction (between 0 and 1).
par3 A flag to switch between up-scattering only ($> 0$) and both up- and down-scattering ($\leq 0$)