Annihilation of heavy-neutral-fermion pairs into monochromatic γ rays and its astrophysical implications

We present estimates for the rates of the processes ξξ yields γγ and ξξ yields Vγ, where ξ is a heavy-neutral-fermion candidate for galactic cold dark matter and V is a vector quarkonium state J/psi(3097) or Υ(9460). A discrepancy with a previous estimate of the rate of the photino pair annihilation process gamma gamma yields γγ 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