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The Astrophysical Journal Letters


The distance of a Fermi-detected blazar γ-ray emission site from a supermassive black hole is a matter of active debate. Here we present a method for testing if the GeV emission of powerful blazars is produced within the subparsec-scale broad-line region (BLR) or farther out in the parsec-scale molecular torus (MT) environment If the GeV emission takes place within the BLR, the inverse Compton (IC) scattering of the BLR ultraviolet (UV) seed photons that produces the γ-rays takes place at the onset of the Klein–Nishina regime. This causes the electron cooling time to become practically energy-independent and the variation of the γ-ray emission to be almost achromatic. If, on the other hand, the γ-ray emission is produced farther out in the parsec-scale MT, the IC scattering of the infrared (IR) MT seed photons that produces the γ-rays takes place in the Thomson regime, resulting in energy-dependent electron cooling times, manifested as faster cooling times for higher Fermi energies. We demonstrate these characteristics and discuss the applicability and limitations of our method.



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