  There has been interest recently on particle physics models that may give
rise to sharp gamma ray spectral features from dark matter annihilation.
Because dark matter is supposed to be electrically neutral, it is challenging
to build weakly interacting massive particle models that may accommodate both a
large cross section into gamma rays at, say, the Galactic center, and the right
dark matter abundance. In this work, we consider the gamma ray signatures of a
class of scalar dark matter models that interact with Standard Model dominantly
through heavy vector-like fermions (the vector-like portal). We focus on a real
scalar singlet S annihilating into lepton-antilepton pairs. Because this
two-body final-state annihilation channel is d-wave suppressed in the chiral
limit, we show that virtual internal bremsstrahlung emission of a gamma ray
gives a large correction, both today and at the time of freeze-out. For the
sake of comparison, we confront this scenario to the familiar case of a
Majorana singlet annihilating into light lepton-antilepton pairs, and show that
the virtual internal bremsstrahlung signal may be enhanced by a factor of (up
to) two orders of magnitude. We discuss the scope and possible generalizations
of the model.
