  The neutralino pair annihilation into two photons in our galactic halo gives
a robust dark matter signal, since it would give a quasi-monotonic gamma ray.
This process is radiatively-induced, and the full-one loop calculation was done
previously. However, for the heavy wino-like or Higgsino-like neutralino, the
one-loop cross section violates unitarity, therefore the higher-order
corrections may be important. We construct a non-relativistic theory for
chargino and neutralino two-body states, and estimate all-order QED corrections
and two-loop corrections by $Z$ and/or $W$ exchange. We find that the critical
mass, above that the two-loop contribution is larger than one-loop one, is
about 8 TeV (O(10) TeV) in the limit where neutralino is wino (Higgsino)-like,
respectively. Around and above the critical mass, the all-order Z and/or W
exchange must be included to estimate the cross section. On the other hand, the
QED corrections depend on the mass difference between the neutralino and
chargino. In the wino-like limit where neutralino is highly degenerate with
chargino in mass, we find that QED corrections enhance the pair annihilation
cross section by 1.5-2.
