Magnetic dark matter for the X-ray line at 3.55 keV
  We consider a decaying magnetic dark matter explaining the X-ray line at 3.55
keV shown recently from XMM-Newton observations. We introduce two singlet
Majorana fermions that have almost degenerate masses and fermion-portal
couplings with a charged scalar of weak scale mass. In our model, an
approximate Z_2 symmetry gives rise to a tiny transition magnetic moment
between the Majorana fermions at one loop. The heavier Majorana fermion becomes
a thermal dark matter due to the sizable fermion-portal coupling to the SM
charged fermions. We find the parameter space for the masses of dark matter and
charged scalar and their couplings, being consistent with both the relic
density and the X-ray line. Various phenomenological constraints on the model
are also discussed.
