Beyond Isotropic Gravitational Waves: Search for Anisotropies with Pulsar Timing Arrays

Recent analyses by pulsar timing arrays (PTAs) have provided evidence for the detection of a stochastic gravitational wave background (SGWB). Traditionally, the SGWB is assumed to be both isotropic and unpolarized. However, anisotropies and polarization in the SGWB could encode rich information about the early Universe and potential sources of gravitational waves. The Overlap Reduction Function (ORF) curve, a key diagnostic in PTA analyses, characterizes the expected correlations between pairs of pulsars caused by an isotropic, unpolarized SGWB. In this work, we apply a generalization of the ORF, as proposed in [1], to search for anisotropic and polarized components in the NANOGrav 15-year data set. Our results may provide insights into the nature of the SGWB and the potential of PTAs as probes of fundamental physics.