A 3.55 keV Line from Exciting Dark Matter without a Hidden Sector
  Models in which dark matter particles can scatter into a slightly heavier
state which promptly decays to the lighter state and a photon (known as
eXciting Dark Matter, or XDM) have been shown to be capable of generating the
3.55 keV line observed from galaxy clusters, while suppressing the flux of such
a line from smaller halos, including dwarf galaxies. In most of the XDM models
discussed in the literature, this up-scattering is mediated by a new light
particle, and dark matter annihilations proceed into pairs of this same light
state. In these models, the dark matter and mediator effectively reside within
a hidden sector, without sizable couplings to the Standard Model. In this
paper, we explore a model of XDM that does not include a hidden sector.
Instead, the dark matter both up-scatters and annihilates through the near
resonant exchange of a $\mathcal{O}(10^2)$ GeV pseudoscalar with large Yukawa
couplings to the dark matter and smaller, but non-neglibile, couplings to
Standard Model fermions. The dark matter and the mediator are each mixtures of
Standard Model singlets and $SU(2)_W$ doublets. We identify parameter space in
which this model can simultaneously generate the 3.55 keV line and the
gamma-ray excess observed from the Galactic Center, without conflicting with
constraints from colliders, direct detection experiments, or observations of
dwarf galaxies.
