The Gravitational Wave Peep and Its Implication for LISA Signal Confusion Noise

Scattering events around the center of massive galaxies will occasionally toss a stellar-mass compact object into an orbit around the massive black hole (MBH) at the center, beginning an extreme mass ratio inspiral (EMRI). The early stages of such a highly eccentric orbit are not likely to produce detectable gravitational waves, as the source will only be in a suitable frequency band briefly when it is close to periapsis during each long-period orbit. This repeated burst of emission, firmly in the millihertz band, is the gravitational wave peep. While a single peep is not likely to be detectable, if we consider an ensemble of such subthreshold sources, spread across the universe, together they may produce an unresolvable background noise that could obscure sources otherwise detectable by LISA. Previous studies of EMRI signal confusion noise focused either on parabolic orbits near the massive black hole or events close to merger. We seek to improve this characterization by implementing numerical kludge waveforms that can calculate highly eccentric orbits. Our focus is on orbits at the point of capture that are less likely to be detectable on their own but will otherwise contribute to the background. We then use estimations of EMRI capture parameters along with tracking the MBH population from a redshift of z=0 to z=3 using the Illustris Project. This information is combined with an estimate of the number of EMRI mergers per unit volume to obtain the number of events contributing to the signal confusion noise.