Charting the nanohertz gravitational wave sky: rapid mapping using the PTA per-frequency optimal statistic

Pulsar timing array collaborations around the world are now reporting evidence for the correlation signature produced by a gravitational wave background (GWB). However, the origin of this background is not yet known, prompting these collaborations to shift their focus towards source characterization. One key method for differentiating an astrophysical GWB from a cosmological GWB is looking for anisotropy. Astrophysical GWBs should produce anisotropies which map the distribution of massive black-hole binaries in the universe and should be unique for every frequency. Despite this focus, current pipelines struggle either with frequency independence or computational complexity. In this talk, we present a new method which leverages the recent advances in the frequentist GWB analysis pipeline called the PTA optimal statistic. By using these advancements, we model and search each frequency independently and include effects from recently derived pulsar pair covariance, an important factor for parameter estimation in the now stronger signal regime PTAs are in. We show that this new pipeline better estimates anisotropy parameters and allows for frequency separation while maintaining the computational speed advantage compared with equivalent Bayesian methods.