  The photomeson production in ultra-high-energy cosmic-ray (UHECR)
accelerators such as gamma-ray bursts and active galaxies may lead to
ultra-high-energy (UHE) gamma-ray emission. We show that generation of UHE
pairs in magnetized structured regions embedding the sources is inevitable, and
accompanied >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 with
~Mpc. Detections will be useful for knowing structured extragalactic magnetic
fields as well as properties of the sources.
