Detecting gravitational wave anisotropies from supermassive black hole binaries

Several pulsar timing array (PTA) collaborations have recently found evidence for a gravitational wave background (GWB) permeating our Galaxy. The origin of this background is still unknown. Indeed, while the gravitational wave emission from inspiraling supermassive black hole binaries (SMBHB) is the primary candidate for its origin, several cosmological sources have also been proposed. One distinctive feature of SMBHB-generated backgrounds is the presence of GWB anisotropies stemming from the binaries distribution in the local Universe. However, none of the anisotropy searches performed to date reported a detection. In this work, we show that the lack of anisotropy detection is not currently in tension with a SMBHB origin of the background. We accomplish this by calculating the probability for present and future PTAs to observe deviations from an isotropic GWB. We find that a PTA with the noise characteristics of the NANOGrav 15-year data set had only a 2%–11% probability of detecting deviations from isotropy in an SMBHB-generated GWB, depending on the properties of the SMBHB population. However, we estimate that for the IPTA DR3 data set these probabilities will increase to 4%–28%, putting more pressure on the SMBHB interpretation in case of a null detection. We also identify SMBHB populations that are more likely to produce significant deviations from isotropy. This information could be used together with the spectral properties of the GWB to characterize the SMBHB population.