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An observation of a redshift-dependent effect does not guarantee actual absorption by EBL background. There would be a possibility that spectral evolution of gamma-ray blazars might coincidentally mimic EBL attenuation. For example, if blazars that formed in the early universe suffered more internal attenuation than younger blazars, a similar effect could be observed. Such possibility has been proposed by Anita Reimer [11] after modeling the intrinsic absorption of gamma-rays with photons from the accretion disk and broad-line region of blazars during periods of strong accretion. Given the blazar emission model considered in her study, and assuming a correlation between accretion history and black hole mass, Anita found that the intrinsic opacity of blazars is redshift-dependent (through black hole mass evolution), and thus, it mimics EBL attenuation.
The If this is the case for the majority of blazars to be observed by GLAST, the intrinsic energy cut-offs blazars are likely to vary significantly blazar-to-blazar, and thus, the energy cut-offs for blazars in a given redshift bin would have larger scattering with respect to the mean than
in EBL-only absorption scenarios. This would allow at least an upper limit on EBL attenuation by looking at the least-attenuated energy-cutoff in a particular redshift bin. Furthermore, intrinsic opacity is likely to change within each blazar during different emission states, allowing thus to constrain over time the nature of the observed energy cutoff.
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