  We provide precise predictions for the hard photon spectrum resulting from
neutral SU$(2)_W$ triplet (wino) dark matter annihilation. Our calculation is
performed utilizing an effective field theory expansion around the endpoint
region where the photon energy is near the wino mass. This has direct relevance
to line searches at indirect detection experiments. We compute the spectrum at
next-to-leading logarithmic (NLL) accuracy within the framework established by
a factorization formula derived previously by our collaboration. This allows
simultaneous resummation of large Sudakov logarithms (arising from a restricted
final state) and Sommerfeld effects. Resummation at NLL accuracy shows good
convergence of the perturbative series due to the smallness of the electroweak
coupling constant - scale variation yields uncertainties on our NLL prediction
at the level of $5\%$. We highlight a number of interesting field theory
effects that appear at NLL associated with the presence of electroweak symmetry
breaking, which should have more general applicability. We also study the
importance of using the full spectrum as compared with a single endpoint bin
approximation when computing experimental limits. Our calculation provides a
state of the art prediction for the hard photon spectrum that can be easily
generalized to other DM candidates, allowing for the robust interpretation of
data collected by current and future indirect detection experiments.
