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

We present estimates for the rates of the processes xixi/r arrow/..gamma gamma..and xixi/r arrow//ital V/..gamma.., where xi is a heavy-neutral-fermion candidatefor galactic cold dark matter and /ital V/ is a vector quarkonium state/ital J///psi/(3097) or UPSILON(9460). A discrepancy with a previous estimateof the rate of the photino pair annihilation process/gamma/tilde ///gamma/tilde ///r arrow/..gamma gamma.. is uncovered and its origin discussed. We explorethe possibility that some of the processes considered may be astrophysicallysignificant sources of monochromatic ..gamma..-ray lines, in the energy rangeof a few to a few tens of GeV, for which theoretical estimates of thebackground ..gamma..-ray flux are unfortunately subject to large uncertainties.