Spectral production rate and number density of the cascade in Mrk 501. The product of the squared dimensionless photon energy with the spectral production rate of photons is plotted in red (which is determined by the last term of Eq. (A.7)). We show the product of the squared dimensionless photon energy with the spectral number density of photons in green (which is determined by Eq. (1)). Both quantities are plotted vs. the product of the highly energetic photon energy with the energy of the helium II Lyman-α line.