The MeerKAT Pulsar Timing Array Maps of the gravitational wave sky with the 4.5-yr data release

In an accompanying publication, the MeerKAT Pulsar Timing Array (MPTA) Collaboration reports tentative evidence for the presence of a stochastic gravitational wave background, following observations of similar signals from the European and Indian Pulsar Timing Arrays, the North American Nanohertz Observatory for Gravitational Waves, the Parkes Pulsar Timing Array, and the Chinese Pulsar Timing Array. If such a gravitational wave background signal originates from a population of inspiraling supermassive black hole binaries, the signal may be anisotropically distributed in the sky. In this paper, we evaluate the anisotropy of the MPTA signal using a spherical harmonic decomposition. We discuss complications arising from the covariance between pulsar pairs and the regularization of the Fisher matrix. Applying our method to the 4.5-yr data set, we obtain two forms of sky maps for the three most sensitive MPTA frequency bins between 7 and 21 nHz⁠. Our ‘clean maps’ estimate the distribution of gravitational wave strain power with minimal assumptions. Our radiometer maps answer the question: Is there a statistically significant point source? We find a noteworthy hotspot in the 7 nHz clean map with a p-factor of p = 0.015 (not including trial factors). Future observations are required to determine if this hotspot is of astrophysical origin.