Fig. 5.
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Doppler factor as function of viewing angle plotted for different bulk Lorentz factors (red Γj = 15, yellow Γj = 10, green Γj = 5, and blue Γj = 2). The ranges of Doppler factors for which the optical depth for γγ-pair production constrained by the variability time scale is smaller than 1 is indicated with the black box and the arrow.
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