  We study spectral features in the gamma-ray emission from dark matter (DM)
annihilation in the Next-to-Minimal Supersymmetric Standard Model (NMSSM), with
either neutralino or right-handed (RH) sneutrino DM. We perform a series of
scans over the NMSSM parameter space, compute the DM annihilation cross section
into two photons and the contribution of box-shaped features, and compare them
with the limits derived from the Fermi-LAT search for gamma-ray lines using the
latest Pass 8 data. We implement the LHC bounds on the Higgs sector and on the
masses of supersymmetric particles as well as the constraints on low-energy
observables. We also consider the recent upper limits from the Fermi-LAT
satellite on the continuum gamma-ray emission from dwarf spheroidal galaxies
(dSphs). We show that in the case of the RH sneutrino the constraint on
gamma-ray spectral features can be more stringent than the dSph bounds. This is
due to the Breit-Wigner enhancement near the ubiquitous resonances with a CP
even Higgs and the contribution of scalar and pseudoscalar Higgs final states
to box-shaped features. By contrast, for neutralino DM, the di-photon final
state is only enhanced in the resonance with a $Z$ boson and box-shaped
features are even more suppressed. Therefore, the observation of spectral
features could constitute a discriminating factor between both models. In
addition, we compare our results with direct DM searches, including the
SuperCDMS and LUX limits on the elastic DM-nucleus scattering cross section and
show that some of these scenarios would be accessible to next generation
experiments. Thus, our findings strengthen the idea of complementarity among
distinct DM search strategies.
