Beyond the Background: Gravitational Wave Anisotropy and Continuous Waves from Supermassive Black Hole Binaries

Emiko C. Gardiner, Luke Zoltan Kelley, Anna-Malin Lemke, Andrea Mitridate

Published 2023-09-13Version 1

Pulsar timing arrays have found evidence for a low-frequency gravitational wave background (GWB). The next gravitational wave (GW) signals astronomers anticipate are Continuous Waves (CWs) from single supermassive black hole binaries (SMBHB) and their associated GWB anisotropy. The prospects for detecting CWs and anisotropy are highly dependent on the astrophysics of SMBHB populations. Thus, information from single sources can break degeneracies in astrophysical models and place more stringent constraints than the GWB alone. We simulate and evolve populations of SMBHBs, model their GWs, and calculate the corresponding detection statistics and levels of anisotropy. We investigate how varying components of our semi-analytic model, including the galaxy stellar mass function, the SMBH-host galaxy relation ($M_{BH}-M_{bulge}$), and the binary evolution prescription impact the expected number of CW detections. This CW occurrence rate is greatest for few total galaxies, high galaxy masses, large scatter and normalization in the galaxy-SMBHB mass relation, and long binary hardening times. The occurrence rate depends most on the binary evolution parameters, implying that CWs offer a novel avenue to constrain binary evolution models. The most detectable CW sources are in the lowest frequency bin, have masses of ~$10^9-10^{10}M_\odot$, and are ~$10^3$ Mpc away. The level of anisotropy increases with frequency, with the angular power spectrum over multipole modes $\ell$ varying in low-frequency $C_{\ell>0}/C_0$ from ~$5\times10^{-3}$ to ~$2\times10^{-1}$ depending on the model; typical values are near current Bayesian upper limits. This anisotropy is correlated with the expected number of CW detections and is fully captured by modeling the ten loudest sources in each frequency bin. Observing this anisotropy would support SMBHB models for the GWB over cosmological models, which tend to be isotropic.