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This paper reports results from numerical simulations of the gravitationalradiation emitted from non--rotating compact objects(both neutron stars andSchwarzschild black holes) as a result of the accretion of matter. A hybridprocedure is adopted: we evolve, in axisymmetry, the linearized equationsdescribing metric and fluid perturbations, coupled with a nonlinearhydrodynamics code that calculates the motion of the accreting matter. Theinitial matter distribution is shaped in the form of extended quadrupolarshells of dust or perfect fluid. Self--gravity and radiation reaction effectsof the accreting fluid are neglected. This idealized setup is used tounderstand the qualitative features appearing in the energy spectrum of thegravitational wave emission from compact stars or black holes, subject toaccretion processes involving extended objects. A comparison for the case ofpoint--like particles falling radially onto black holes is also provided. Ourresults show that, when the central object is a black hole, the spectrum is farfrom having only one clear, monochromatic peak at the frequency of thefundamental quasi-normal mode, but it shows a complex pattern, withinterference fringes produced by the interaction between the infalling matterand the underlying perturbed spacetime: most of the energy is emitted atfrequencies lower than that of the fundamental mode of the black hole. Similarresults are obtained for extended shells accreting onto neutron stars, but inthis case the stellar fundamental mode is clearly excited. Our analysis showsthat the gravitational wave signal driven by accretion is influenced more bythe details and dynamics of the process, than by the quasi--normal modestructure of the central object.Comment: 23 pages, 12 figures, v3. Replaced fig.6. Small changes in the text  to match the version published in Phys. Rev.

Accretion-driven gravitational radiation from nonrotating compact objects: Infalling quadrupolar shells