step: step function convolved with gaussian

A step function convolved with a gaussian.


\begin{displaymath}
A(E) = {K\over{2}}[1-\mbox{erf}({(E-E_s)\over{\sqrt{2}\sigma}})]
\end{displaymath}

par1 $E_s$, start energy (keV)
par2 $\sigma$, gaussian sigma (keV)
norm $K$, step amplitude