The caustic technique uses galaxy redshifts alone to measure the escape velocity and mass profiles of galaxy clusters to clustrocentric distances well beyond the virial radius, where dynamical equilibrium does not necessarily hold. We provide a detailed description of this technique and analyse its possible systematic errors. We apply the caustic technique to clusters with mass M200≥ 1014 h−1 M⊙ extracted from a cosmological hydrodynamic simulation of a ΛCDM universe. With a few tens of redshifts per squared comoving megaparsec within the cluster, the caustic technique, on average, recovers the profile of the escape velocity from the cluster with better than 10 per cent accuracy up to r∼ 4r200. The caustic technique also recovers the mass profile with better than 10 per cent accuracy in the range (0.6–4) r200, but it overestimates the mass up to 70 per cent at smaller radii. This overestimate is a consequence of neglecting the radial dependence of the filling function . The 1σ uncertainty on individual escape velocity profiles increases from ∼20 to ∼50 per cent when the radius increases from r∼ 0.1r200 to ∼4r200. Individual mass profiles have 1σ uncertainty between 40 and 80 per cent within the radial range (0.6–4) r200. When the correct virial mass is known, the 1σ uncertainty reduces to a constant 50 per cent on the same radial range. We show that the amplitude of these uncertainties is completely due to the assumption of spherical symmetry, which is difficult to drop. Other potential refinements of the technique are not crucial. We conclude that, when applied to individual clusters, the caustic technique generally provides accurate escape velocity and mass profiles, although, in some cases, the deviation from the real profile can be substantial. Alternatively, we can apply the technique to synthetic clusters obtained by stacking individual clusters: in this case, the 1σ uncertainty on the escape velocity profile is smaller than 20 per cent out to 4r200. The caustic technique thus provides reliable average profiles which extend to regions difficult or impossible to probe with other techniques.

Cluster mass profiles are tests of models of structure formation. Only two current observational methods of determining the mass profile, gravitational lensing, and the caustic technique are independent of the assumption of dynamical equilibrium. Both techniques enable the determination of the extended mass profile at radii beyond the virial radius. For 19 clusters, we compare the mass profile based on the caustic technique with weak lensing measurements taken from the literature. This comparison offers a test of systematic issues in both techniques. Around the virial radius, the two methods of mass estimation agree to within ∼30%, consistent with the expected errors in the individual techniques. At small radii, the caustic technique overestimates the mass as expected from numerical simulations. The ratio between the lensing profile and the caustic mass profile at these radii suggests that the weak lensing profiles are a good representation of the true mass profile. At radii larger than the virial radius, the extrapolated Navarro, Frenk & White fit to the lensing mass profile exceeds the caustic mass profile. Contamination of the lensing profile by unrelated structures within the lensing kernel may be an issue in some cases; we highlight the clusters MS0906+11 and A750, superposed along the line of sight, to illustrate the potential seriousness of contamination of the weak lensing signal by these unrelated structures.