  The decay of dark matter particles which are coupled predominantly to charged
leptons has been proposed as a possible origin of excess high-energy positrons
and electrons observed by cosmic-ray telescopes PAMELA and Fermi LAT. Even
though the dark matter itself is electrically neutral, the tree-level decay of
dark matter into charged lepton pairs will generically induce radiative
two-body decays of dark matter at the quantum level. Using an effective theory
of leptophilic dark matter decay, we calculate the rates of radiative two-body
decays for scalar and fermionic dark matter particles. Due to the absence of
astrophysical sources of monochromatic gamma rays, the observation of a line in
the diffuse gamma-ray spectrum would constitute a strong indication of a
particle physics origin of these photons. We estimate the intensity of the
gamma-ray line that may be present in the energy range of a few TeV if the dark
matter decay interpretation of the leptonic cosmic-ray anomalies is correct and
comment on observational prospects of present and future Imaging Cherenkov
Telescopes, in particular the CTA.
