A geometric multimessenger consistency test of radiative and near-zone gravity with LISA and SKA

Compact binary pulsars observed both through precision radio timing and low-frequency gravitational waves offer a direct way to compare the same binary geometry with two independent messengers. We propose a multimessenger consistency test based on the orbital inclination, measured from the Shapiro-delay shape parameter in radio timing and from the tensor polarization amplitude ratio in the gravitational-wave signal. Defining the common-epoch residual (t 0)=s Shapiro (t 0)-s GW (t 0), general relativity predicts =0, while a nonzero value would indicate either an unmodeled systematic or a mismatch between the near-zone and radiative descriptions of gravity. We estimate the attainable precision on this quantity for representative LISA--SKA compact binary pulsars using a seven-parameter timing Fisher matrix and a sky-averaged LISA sensitivity curve including the Galactic foreground. We adopt a conservative radio baseline, sigma TOA=1, mu s and N eff=10 4, intended to summarize radiometer noise, jitter, residual dispersion-measure and scattering effects, profile evolution, and cadence losses after wideband timing. For systems at d=5, kpc observed for four years, we find sigma 4 times10-3 for a favorable double neutron star and sigma 9 times10-4 for a hypothetical pulsar--black-hole system. The former is the more robust astrophysical benchmark; the latter illustrates the reach if such a high-SNR chirping source is discovered. The useful cases remain limited mainly by gravitational-wave polarimetry, while radio timing supplies the near-zone reference measurement of the inclination. These results define a quantitative target for future joint Bayesian analyses of compact binary pulsars observed in both radio and gravitational waves.