Gravitational signals emitted by a point mass orbiting a neutron star: A perturbative approach - art. no. 104007

We compute the energy spectra of the gravitational signals emitted when a p ointlike mass moves on a closed orbit around a nonrotating neutron star, in ducing a perturbation of its gravitational field and its internal structure . The Einstein equations and the hydrodynamical equations are perturbed and numerically integrated in the frequency domain. The results are compared w ith the energy spectra computed by the quadrupole formalism which assumes t hat both masses are pointlike, and accounts only for the radiation emitted because the orbital motion produces a time dependent quadrupole moment. The results of our perturbative approach show that, in general, the quadrupole formalism overestimates the amount of emitted radiation, especially when t he two masses are close. However, if the pointlike mass is allowed to move on an orbit so tight that the Keplerian orbital frequency resonates with th e frequency of the fundamental quasinormal mode of the star (2 omega (K) = omega (f)), this mode can be excited and the emitted radiation can be consi derably larger than that computed by the quadrupole approach.