Time-Correlated Profile Variability in the MeerKAT Pulsar Timing Array
Long-term pulse-profile variability has emerged as a potentially important source of uncertainty for high-precision pulsar timing, as changes in the integrated pulse shape can bias pulse times of arrival and affect the sensitivity of pulsar timing arrays to nanohertz gravitational waves. We present a systematic search for long-term pulse-profile variability in the six-year MeerKAT Pulsar Timing Array data, analysing observations of 84 millisecond pulsars using a two-dimensional Gaussian-process framework to identify coherent temporal and phase-dependent profile evolution. We detect pulse-profile variability in 18 pulsars. Using the frequency dependence of the observed profile evolution together with scattering simulations, we classify six pulsars as exhibiting chromatic variability consistent with interstellar scattering, while the remaining twelve display predominantly frequency-independent behaviour suggestive of intrinsic changes in the pulsar emission process. Correcting for the S/N-dependent detection bias, we infer that intrinsic profile variability is substantially more common than implied by the detected sample, with at least 40 per cent of millisecond pulsars exhibiting intrinsic profile variability at the 95 per cent credible level. Motivated by the association between pulse-profile variability and spin-down-rate ($\dot{\nu}$) switching in canonical pulsars, we search for corresponding $\dot{\nu}$ variations but find no statistically significant detections. Simulations indicate that the expected variations remain below the sensitivity of current datasets, although unresolved changes may contribute to weak achromatic timing noise. These results suggest that long-term pulse-profile variability may be common among millisecond pulsars and should be explicitly modelled in future high-precision pulsar-timing experiments.