powerlaw, zpowerlw: power law photon spectrum

powerlaw is a simple photon power law. The zpowerlw variant computes a redshifted spectrum.

$$A(E) = K E^{-\alpha}$$

where:

par1	$\alpha$, photon index of power law (dimensionless)
norm	$K$, photons/keV/cm$^2$/s at 1 keV

For zpowerlw the formula and corresponding parameters are:

$$A(E) = K [E(1+z)]^{-\alpha}$$

where:

par1	$\alpha$, photon index of power law (dimensionless)
par2	$z$, redshift
norm	$K$, photons/keV/cm$^2$/s at 1 keV

If POW_EMIN and POW_EMAX have been defined by the xset command then the norm becomes the flux in units of $10^{-12}$ ergs/cm$^2$/s over the energy range (POW_EMIN, POW_EMAX) keV unless POW_EMIN = POW_EMAX in which case the norm becomes the flux density in micro-Jansky at POW_EMIN keV. In these cases it is important that POW_EMIN and POW_EMAX lie within the energy range on which the model is being evaluated.