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Optical depth by photon-photon collision as a function of the photon energy for sources located at z = 0.003, 0.01, 0.03, 0.1, 0.3, 0.5, 1, 1.5, 2, 2.5, 3, 4, from bottom to top. Black and red lines are from the present work and from AF2008, respectively. For the reader’s convenience, the thick black line marks the z = 1 line.
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