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polpow: Power-law dependent polarization

This multiplicative model applies a polarization with a power-law dependence on energy. The factor applied depends on which Stokes parameter the spectrum is for. The Stokes parameter is determined using an XFLTnnnn keyword with values ``Stokes:0'', ``Stokes:1'', or ``Stokes:2'', for I, Q, U, respectively.

The fraction and angle are determined by

\begin{displaymath}A(E) = {\tt Anorm} \times E^{-{\tt Aindex}}\end{displaymath}

\begin{displaymath}\psi(E) = {\tt psinorm} \times E^{-{\tt psiindex}}\end{displaymath}

The parameters are :

par1 Anorm, the polarization fraction at 1 keV
par2 Aindex, the polarization fraction index
par3 psinorm, the polarization angle at 1 keV (degrees)
par4 psiindex, the polarization angle index