A forest of gravitational waves in our Galactic Centre

Abstract: At the Galactic Centre, we can expect a population of a few tens of early extreme-mass ratio inspirals (E-EMRIs) and extremely large mass ratio inspirals (XMRIs). Depending on their evolutionary stage, they can be highly eccentric, with moderate signal-to-noise ratios (SNRs) of tens or hundreds, or nearly circular, with SNRs as large as a few thousand. Their individual signals combine into a common signal, which can complicate the resolution of other types of sources. We have calculated the foreground signal of continuous E-EMRIs and XMRIs using a catalog based on the expected number of sources and a realistic phase-space distribution. The forest of E-EMRIs will cover a large portion of the LISA sensitivity curve, obscuring the signals of some massive black hole binaries, verification binaries, and harmonics of EMRIs in their polychromatic phase. The combined signal from XMRIs will be much weaker but still affect intermediate-mass black hole binaries. Due to the large SNR, this forest can be also found in other galactic nuclei, such as that of the Andromeda galaxy. Even under conservative assumptions, the forest created by E-EMRIs and XMRIs in our Galactic Centre will likely pose a challenge for resolving other types of sources, as their contribution is non-coherent and exhibits large SNRs.