...
Figure 2 presents a scatter plot of the energy cutoff vs redshift obtained from the analysis of the Monte Carlo simulation. This relation was first introduced by Fazio & Stecker in 1970 [10] as a way to relate the energy
cutoff with the redshift of the source. Kneiske et al 9 have proposed to use the Fazio-Stecker relation (FSR) to compare EBL models with the FSR distribution obtained from observations. This idea is implemented here by considering the FSR obtained after determination of the cutoff energies of the brightest blazars expected to be observed with GLAST. The black squares indicate the energy cut-offs as determined from the fits (observations) and can be seen to reproduce very well the EBL model used for the simulation (Kneiske et al's "Best Fit"). Not all the sources considered in the simulation produced meaningful fits: for some blazars the error in the determination of the energy cutoff is greater than the value itself. This is due to the lack of photons at the highest energies for sources with soft intrinsic spectra (index > 2.5). Of the 165 blazars included in the simulation with redshift z > 0.5 (i.e. with EBL energy cut-offs that are in the energy range measured by GLAST) 97 of them yielded meaningful fits.
In the absence of blazar intrinsic absorption and strong blazar spectrum curvature, the data points in the FSR plot will converge (amid statistical fluctuations) to the true curve due to EBL absorption. If it turns out, however, that this is not the case for a few or most blazars, their measured cut-off energies would spread below the EBL-induced value, but never above. This would enable at least and upper limit on EBL attenuation (least-attenuated flux in a particular redshift range).
...