Gravitational waves from nonradial oscillations of stochastically accreting neutron stars

Abstract

Abstract Oscillating neutron stars are sources of continuous gravitational waves. We study analytically the excitation of stellar oscillations by the mechanical impact on the stellar surface of ‘clumps’ of stochastically accreted matter. We calculate the waveform and spectrum of the gravitational wave signal emitted by the accretion-driven pulsations. Results are generated for an idealised model of a nonrotating, unmagnetised, one-component star with uniform polytropic index npoly assuming Newtonian gravity and the Cowling approximation. We find that the excited mode amplitudes grow with increasing npoly and mode order n. The gravitational wave signal forms a sequence of amplitude-modulated packets for npoly = 1, lasting ∼10−3s after each impact. The gravitational wave strain increases with increasing npoly, but decreases with increasing n and increasing multipole order l for npoly = 1. In the observing band of current long-baseline interferometers, g-modes emit higher, narrower peaks in the amplitude spectral density than f- and p-modes, with the highest peaks reaching ∼10−26Hz−1/2 for modes with damping time τnl ∼ 108yr. The root-mean-square strain hrms, calculated by summing over modes with 2 ≤ l ≤ 4 and τnl ≤ 108yr, spans the range 10−33 ≤ hrms ≤ 10−32 for 1 ≤ npoly ≤ 2.