Toward Robust Detections of Nanohertz Gravitational Waves

Abstract

Abstract The recent observation of a common red-noise process in pulsar timing arrays (PTAs) suggests that the detection of nanohertz gravitational waves might be around the corner. However, in order to confidently attribute this red process to gravitational waves, one must observe the Hellings–Downs curve—the telltale angular correlation function associated with a gravitational-wave background. This effort is complicated by the complex modeling of pulsar noise. Without proper care, misspecified noise models can lead to false-positive detections. Background estimation using “quasi-resampling” methods such as sky scrambles and phase shifts, which use the data to characterize the noise, are therefore important tools for assessing significance. We investigate the ability of current PTA experiments to estimate their background with “quasi-independent” scrambles—characterized by a statistical “match” below the fiducial value: ∣M∣ < 0.1. We show that sky scrambling is affected by “saturation” after  ( 10 ) quasi-independent realizations; subsequent scrambles are no longer quasi-independent. We show that phase scrambling saturates after  ( 100 ) quasi-independent realizations. With so few independent scrambles, it is difficult to make reliable statements about the ≳5σ tail of the null distribution of the detection statistic. We sketch out various methods by which one may increase the number of independent scrambles. We also consider an alternative approach, wherein one reframes the background estimation problem so that the significance is calculated using statistically dependent scrambles. The resulting p-value is in principle well defined, but may be susceptible to failure if assumptions about the data are incorrect.