Speaker
Description
An important next step for pulsar timing arrays (PTAs) is to measure anisotropies in the stochastic gravitational wave background (SGWB) at $\sim$ nano-Hz frequencies. We present simple analytical estimates of the anisotropy signals in PTA, considering contributions from both shot-noise and large-scale structure (LSS), under the assumption that PTA signals primarily originate from the inspiral phase of supermassive black hole binaries (SMBHBs). We find that shot-noise-induced signal dominates over the LSS-induced one and exhibits a steep frequency dependence compared to the isotropic background signals, with $C_{\ell>0,h^2}^{\rm SN} \propto f^{8/3}$. At high frequencies, shot-noise anisotropies can be close to the background and even naively surpass the current NANOGrav constraints. Given the potentially large amplitude of these signals, near-future PTA experiments may be able to detect shot-noise anisotropies, or at least place tighter upper limits, which would already be informative for distinguishing between different SMBHB source configurations and possible source origins. While the correlation between PTA anisotropies and galaxy surveys could, in principle, offer a novel probe of SMBHB evolution, such studies are likely to remain challenging in the near term, as PTA sensitivity is currently limited to large angular scales.
