A broken power law.
where :
par1 = power law photon index for E < break energy par2 = break point for the energy in keV par3 = power law photon index for E > break energy K = photons/keV/cm/s at 1 keV
par2 = break point for the energy in keV
par3 = power law photon index for E > break energy
K = photons/keV/cm/s at 1 keV