Gravitational Wave Peep Contributions to Background Signal Confusion Noise for LISA

Two-body gravitational interactions will occasionally lead to a stellar-mass compact object entering a very highly eccentric orbit around a massive black hole at the center of a galaxy. Gravitational radiation damping will subsequently result in an extreme mass ratio inspiral. Much of the inspiral time of these events is spent with the compact object on a long period orbit, with a brief burst of gravitational wave emission at periapsis firmly in the LISA band, these repeated sources are known as “peeps” which make up most of the amplitude of the full EMRI signal. These burst orbits have been previously modeled as parabolic with a focus on extreme examples that could be detectable by space-based gravitational wave detectors.

However, it is also important to consider them as possible sources of signal confusion noise, because they do generate signal within the LISA band, the result of the “peeps” which recur at every orbit. This project expands those former studies by looking at these repeated bursts, or peeps, and their contribution to the background signal confusion noise for LISA which may obscure otherwise detectable sources in the \(10^{-5}\)–\(10^{-3}\) Hz range. This is done by utilizing estimates for EMRI capture parameters along with tracking the massive black hole population from a redshift of z=0 to z=3 using the Illustris Project. Then, this population is combined with an EMRI formation rate to estimate the number of EMRI events per unit volume for LISA. In this study, we model three different assumptions for the gravitational wave background produced by these highly eccentric peeps. We found that by including only peeps modeled very far away from the massive black hole and early in their orbit did not generate a background that is likely to obscure otherwise detectable sources, however in a more abundant case could produce an unresolveable background.