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

TL;DR

This paper presents sky maps of gravitational-wave strain from the MeerKAT Pulsar Timing Array's 4.5-year data, revealing a potential hotspot that could indicate anisotropic gravitational-wave sources.

Contribution

The paper introduces a method to map gravitational-wave anisotropy using spherical harmonics and applies it to the MeerKAT data, producing the first sky maps at these frequencies.

Findings

Detection of a potential gravitational-wave hotspot at 7 nHz

Development of a new spherical harmonic decomposition method

First sky maps of gravitational-wave strain from MeerKAT data

Abstract

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, NANOGrav, 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 on the sky. In this Letter we evaluate the anisotropy of the MPTA signal using a spherical harmonic decomposition. We discuss complications arising from the covariance between pulsar pairs and regularisation of the Fisher matrix. Applying our method to the 4.5 yr dataset, we obtain two forms of sky maps for the three most sensitive MPTA frequency bins between 7 -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.