bkn2pow: broken power law, 2 break energies

A three-segment broken power law (i.e. with two break energies).

$$A(E) = \left\{ \begin{array}{ll} K E^{-\Gamma_1} & \mbox{if... ..._3} & \mbox{if $E_{break,2} \leq E$} \\ \end{array} \right.$$

where:

par1 = $\Gamma_1$	power law photon index for $E \leq E_{break,1}$
par2 = $E_{break,1}$	first break point for the energy in keV
par3 = $\Gamma_2$	power law photon index for $E_{break,1} \leq E \leq E_{break,2}$
par4 = $E_{break,2}$	second break point for the energy in keV
par5 = $\Gamma_3$	power law photon index for $E > E_{break,2}$
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.