Forecasting pulsar timing array sensitivity to anisotropy in the stochastic gravitational wave background

Abstract: Statistical anisotropy in the nanohertz-frequency gravitational-wave background (GWB) is expected to be detected by pulsar timing arrays (PTAs) in the near future. By developing a frequentist statistical framework that intrinsically restricts the GWB power to be positive, we establish scaling relations for multipole-dependent anisotropy decision thresholds that are a function of the noise properties, timing baselines, and cadences of the pulsars in a PTA. We verify that $(i)$ a larger number of pulsars, and $(ii)$ factors that lead to lower uncertainty on the cross-correlation measurements between pulsars, lead to a higher overall GWB signal-to-noise ratio, and lower anisotropy decision thresholds with which to reject the null hypothesis of isotropy. Using conservative simulations of realistic NANOGrav datasets, we predict that an anisotropic GWB with angular power $C_{l=1} > 0.3\,C_{l=0}$ may be sufficient to produce tension with isotropy at the $p = 3\times10^{-3}$ ($\sim3\sigma$) level in near-future NANOGrav data with a $20$~yr baseline. We present ready-to-use scaling relationships that can map these thresholds to any number of pulsars, configuration of pulsar noise properties, and sky coverage. We discuss how PTAs can improve the detection prospects for anisotropy, as well as how our methods can be adapted for more versatile searches.