2025MNRAS.536.1489M

Pulsar timing arrays search for nanohertz-frequency gravitational waves by regularly observing ensembles of millisecond pulsars over many years to look for correlated timing residuals. Recently the first evidence for a stochastic gravitational wave background has been presented by the major arrays, with varying levels of significance (${\sim} 2\sigma \!-\! 4\sigma$). In this paper, we present the results of background searches with the MeerKAT Pulsar Timing Array. Although of limited duration (4.5 yr), the ${\sim} 250\,000$ arrival times with a median error of just $3 \, \mu {\rm s}$ on 83 pulsars make it very sensitive to spatial correlations. Detection of a gravitational wave background requires careful modelling of noise processes to ensure that any correlations represent a fit to the underlying background and not other misspecified processes. Under different assumptions about noise processes, we can produce either what appear to be compelling Hellings-Downs correlations of high significance ($3\sigma \!-\! 3.4\sigma$) with a spectrum close to that which is predicted, or surprisingly, under slightly different assumptions, ones that are insignificant. This appears to be related to the fact that many of the highest precision MeerKAT Pulsar Timing Array pulsars are in close proximity and dominate the detection statistics. The sky-averaged characteristic strain amplitude of the correlated signal in our most significant model is $h_{{\rm c}, {\rm yr}} = 7.5^{+0.8}_{-0.9} \times 10^{-15}$ measured at a spectral index of $\alpha =-0.26$, decreasing to $h_{{\rm c}, {\rm yr}} = 4.8^{+0.8}_{-0.9} \times 10^{-15}$ when assessed at the predicted $\alpha =-2/3$. These data will be valuable as the International Pulsar Timing Array project explores the significance of gravitational wave detections and their dependence on the assumed noise models.