Beta profiles

The two implemented profile functions can be modified for more refined profiles. The Lorentzian profile in combination with a parameter $\beta$ as exponent is called Beta profile. For Chandra LETGS spectra $\beta=2.5$ represents the instrumental profile better than the Gaussian profile. The implementation is

$$P_{\lambda,\sigma}(x) = \left[\frac{\sigma/2\pi}{[(x-\lambda)^2+\sigma^2/4}\right]^\beta$$ (3)