Internal bremsstrahlung signatures in light of direct dark matter
  searches
  Although proposed long ago, the search for internal bremsstrahlung signatures
has only recently been made possible by the excellent energy resolution of
ground-based and satellite-borne gamma-ray instruments. Here, we investigate
thoroughly the current status of internal bremsstrahlung searches in light of
the results of direct dark matter searches and in the framework of a minimal
mass-degenerate scenario consisting of a Majorana dark matter particle that
couples to a fermion and a scalar via a Yukawa coupling. The upper limits on
the annihilation cross section set by Fermi-LAT and H.E.S.S. extend
uninterrupted from tens of GeV up to tens of TeV and are rather insensitive to
the mass degeneracy in the particle physics model. In contrast, direct searches
are best in the moderate to low mass splitting regime, where XENON100 limits
overshadow Fermi-LAT and H.E.S.S. up to TeV masses if dark matter couples to
one of the light quarks. In our minimal scenario we examine carefully the
prospects for GAMMA-400, CTA and XENON1T, all planned to come online in the
near future, and find that: (a) CTA and XENON1T are fully complementary, with
CTA most sensitive to multi-TeV masses and mass splittings around 10%, and
XENON1T probing best small mass splittings up to TeV masses; and (b) current
constraints from XENON100 already preclude the observation of any spectral
feature with GAMMA-400 in spite of its impressive energy resolution, unless
dark matter does not couple predominantly to light quarks. Finally, we point
out that, unlike for direct searches, the possibility of detecting thermal
relics in upcoming internal bremsstrahlung searches requires, depending on the
concrete scenario, boost factors larger than 5-10.
