We present the Sommerfeld enhanced Dark Matter (DM) annihilation into gamma
ray for a class of three-loop radiative neutrino mass models with large
electroweak multiplets where the DM mass is in O(TeV) range. We show that in
this model, the DM annihilation rate becomes more prominent for larger
multiplets and it is already within the reach of currently operating Imaging
Atmospheric Cherenkov telescopes (IACTs), High Energy Stereoscopic System
(H.E.S.S.). Furthermore, Cherenkov Telescope Array (CTA), which will begin
operating in 2030, will improve this sensitivity by a factor of
O(10) and may exclude a large portion of parameter space of this
radiative neutrino mass model with larger electroweak multiplet. This implies
that the only viable option is the model with lowest electroweak multiplets
i.e. singlets of SU(2)Lβ where the DM annihilation rate is not Sommerfeld
enhanced and hence it is not yet constrained by the indirect detection limits
from H.E.S.S. or future CTA.Comment: 12 pages, 7 figure