Annihilation of heavy-neutral-fermion pairs into monochromaticγrays and its astrophysical implications

We present estimates for the rates of the processes {χ}{χ}{→}{γ}{γ} and {χ}{χ}{→}V{γ}, where {χ} is a heavy-neutral-fermion candidate for galactic cold dark matter and V is a vector quarkonium state J/{ψ}(3097) or {Υ}(9460). A discrepancy with a previous estimate of the rate of the photino pair annihilation process {γ} ̃ \~{}{}{γ} ̃ \~{}{}{→}{γ}{γ} is uncovered and its origin discussed. We explore the possibility that some of the processes considered may be astrophysically significant sources of monochromatic {γ}-ray lines, in the energy range of a few to a few tens of GeV, for which theoretical estimates of the background {γ}-ray flux are unfortunately subject to large uncertainties.