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pegpwrlw: power law, pegged normalization

A power law with pegged normalization.

Using pegpwrlw or cflux * pow should be equivalent. The former is preferred for simplicity.

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

where :

par1 $\alpha $, photon index of power law (dimensionless)
par2 lower peg energy range (keV)
par3 upper peg energy range (keV)
norm flux (in units of $10^{-12}$ ergs/cm$^2$/s over the energy par2par3). If par2 = par3, it is the flux in micro-Jy at par2.