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

*A*(*E*)* = K E*^{–}α

par1 = α |
photon index of power law (dimensionless) |

norm= |
photons
keV |

For **zpowerlw** the formula and
corresponding parameters are:

*A*(*E*)* = K *[*E*(1+*z*)]^{–}α

where :

par1= α |
photon index of power law (dimensionless) |

par2= |
Redshift |

norm= |
photons 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^{-1} 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.