2012ApJ...745L..16M

The photomeson production in ultra-high-energy cosmic-ray (UHECR) accelerators such as γ-ray bursts and active galaxies may lead to ultra-high-energy (UHE) γ-ray emission. We show that the generation of UHE pairs in magnetized structured regions where the sources are embedded is inevitable, and accompanying ≳ 0.1 TeV synchrotron emission provides an important probe of UHECR acceleration. It would especially be relevant for powerful transient sources, and synchrotron pair echoes may be detected by future CTA via coordinated search for transients of duration ∼0.1-1 yr for the structured regions of ∼Mpc. Detections will be useful for knowing structured extragalactic magnetic fields as well as properties of the sources.