  More than a dozen blazars are known to be emitters of multi-TeV gamma rays,
often with strong and rapid flaring activity. By interacting with photons of
the cosmic microwave and infrared backgrounds, these gamma rays inevitably
produce electron-positron pairs, which in turn radiate secondary inverse
Compton gamma rays in the GeV-TeV range with a characteristic time delay that
depends on the properties of the intergalactic magnetic field (IGMF). For
sufficiently weak IGMF, such "pair echo" emission may be detectable by the
Gamma-ray Large Area Space Telescope (GLAST), providing valuable information on
the IGMF. We perform detailed calculations of the time-dependent spectra of
pair echos from flaring TeV blazars such as Mrk 501 and PKS 2155-304, taking
proper account of the echo geometry and other crucial effects. In some cases,
the presence of a weak but non-zero IGMF may enhance the detectability of
echos. We discuss the quantitative constraints that can be imposed on the IGMF
from GLAST observations, including the case of non-detections.

Astrophysics