CLASH-VLT: Constraining deviation from GR with the mass profiles of nine massive galaxy clusters

We investigate the anisotropic stress parameter, $\eta=\Psi/\Phi$, defined as the ratio of the gravitational potentials in the linearly perturbed Friedmann-Lema\^itre Robertson-Walker metric, as a probe of deviations from general relativity across astrophysical to cosmological scales. Using mass profiles reconstructed from high-precision lensing and kinematics of nine galaxy clusters from the CLASH-VLT sample, we derive $\eta(r)$ as a function of the radial distance from the cluster centres, over the range $[0.1 \,\text{Mpc},1.2\,r_{200}^L]$, where $r_{200}^L$ is virial radius best-fit from lensing data. When using a Navarro-Frenk-White or an Hernquist profile to model the total matter distribution, we find consistency with general relativity ($\eta = 1$) within $2\sigma$ for the full radial range for all the sampled clusters. However, adopting a Burkert profile introduces mild tension with general relativity, reaching the $3\sigma$ level in two systems. Assuming a negligible time-dependence in the redshift range spawned by the clusters, we obtain the joint constraint $\eta (r= 1.0 \, \text{Mpc}) = 0.93^{+0.48}_{-0.40}$ (stat) $\pm 0.47$ (syst) at $95\%$ confidence level -- an improvement of approximately $40\%$ over previous estimates. We discuss the impact of systematics on the constraints, and we highlight the implications of this result for current and upcoming cluster surveys.