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tokens: i.e., . The relative plasmoid distances in a string vary depending on their different speeds along the jet, with the leading one significantly slowed down primarily by snow-plow effects against jet material, in particular, BLR clouds. This constitutes an interesting condition as the leading member in a string can reprocess and partially reflect some of the primary S radiation emitted by the trailing members (photons “mirrored” by slow clouds in the BLR were originally proposed by Ghisellini Madau 1996 ). Shrinking mirror-plasmoid gaps so produced provide compact sites with high seed photon densities for intense IC to take place. In a scenario with multiple emitting plasmoids (see Figure 1 ), the primary S flux in the optical–UV bands is emitted by a plasmoid like H starting within the BLR, that is, in the range . The S flux is emitted forward and mirrored backward by the leading plasmoid M toward an incoming plasmoid K . The whole process is subject to a “causality condition” (Böttcher Dermer 1998 ); the mirrored photons are received by an incoming plasmoid when the latter is at the receiving point that we denote with (cf. Figure 1 ). Figure 1. Schematic picture of the source structures we expect at and beyond the BLR. S radiation (orange dashed line) is emitted by inner plasmoids such as H at a radius r h and is reflected back (violet dotted lines) by the outermost mirror-plasmoid M at a radius toward an incoming plasmoid K : the encounter occurs at a radius . Dense seed photons build up in the gap between the approaching plasmoid K and the mirror, where they are IC upscattered to gamma-ray energies (green dotted lines) by electrons accelerated in and around plasmoids. The condition is easily evaluated when the same plasmoid moving with radial coordinate and boost Γ first emits at r , and then receives the radiation reflected backward by the mirror moving with coordinate and boost . The receiving point reads in the AGN frame, for primary photons emitted in the named range and reflected back at /. Here and correspondingly are the relative velocity and boost between the receiver and the mirror (note that /). Moreover, is the receiving point for primary photons emitted at and reflected at /. Note that for small values of r 0 the approximation holds; so the factor makes it possible to have relevant reflections beyond R BLR . The IC radiative event is completed when the plasmoid K crosses the effective gap defined byand lasts for an observer time /: this turns out to be about 2 minutes when, e.g., , , and hold. In the initial stage, the primary emitter radiates from an inner position and the mirror surface is small relative to the cross section of the emission beam as to reduce the effective power received by the mirror to in the head-on approximation and with . A fraction thereof is reflected back and is received by the incoming plasmoid when the latter is close to the reflection point, that is, in a cone of a cross section of order . Then,