  High-energy emission from gamma-ray bursts (GRBs) is widely expected but had
been sparsely observed until recently when the Fermi satellite was launched. If
>TeV gamma rays are produced in GRBs and can escape from the emission region,
they are attenuated by the cosmic infrared background photons, leading to
regeneration of GeV-TeV secondary photons via inverse-Compton scattering. This
secondary emission can last for a longer time than the duration of GRBs, and it
is called a pair echo. We investigate how this pair echo emission affects
spectra and light curves of high energy afterglows, considering not only prompt
emission but also afterglow as the primary emission. Detection of pair echoes
is possible as long as the intergalactic magnetic field (IGMF) in voids is
weak. We find (1) that the pair echo from the primary afterglow emission can
affect the observed high-energy emission in the afterglow phase after the jet
break, and (2) that the pair echo from the primary prompt emission can also be
relevant, but only when significant energy is emitted in the TeV range,
typically E_{gamma, >0.1 TeV} > (Y/(1+Y)) epsilon_e E_k. Even non-detections of
the pair echoes could place interesting constraints on the strength of IGMF.
The more favorable targets to detect pair echoes may be the "naked" GRBs
without conventional afterglow emission, although energetic naked GRBs would be
rare. If the IGMF is weak enough, it is predicted that the GeV emission extends
to >30-300 s.
