Sacha Guerrini

Context. Weak gravitational lensing requires precise measurements of galaxy shapes and therefore accurate knowledge of the point spread function (PSF) model. The latter can be a source of systematics that affect the shear two-point correlation function. A key aspect of weak-lensing analysis is the forecasting of the systematics due to the PSF. Aims. Correlation functions of galaxies and the PSF, the so-called \ensuremathρ and \ensuremathτ statistics, are used to evaluate the level of systematics coming from the PSF model and PSF corrections and contributing to the two-point correlation function used to perform cosmological inference. Our goal is to introduce a fast and simple method to estimate this level of systematics and to assess its agreement with state-of- the-art approaches. Methods. We introduce a new way to estimate the covariance matrix of \ensuremathτ statistics using analytical expressions. The covariance allows us to estimate parameters directly related to the level of systematics associated with the PSF and provides us with a tool to validate the PSF model used in a weak-lensing analysis. We applied these methods to data from the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS). Results. We show that semi-analytical covariance yields results comparable to those obtained by using covariances obtained from simulations or jackknife resampling. The approach requires less computation time and is therefore well suited to rapid comparison of the systematic level obtained from different catalogues. We also show how one can break degeneracies between parameters with a redefinition of the \ensuremathτ statistics. Conclusions. The methods developed in this work will be useful tools in the analysis of current weak-lensing data but also of Stage IV surveys such as Euclid, LSST, and Roman. They provide fast and accurate diagnostics on PSF systematics that are crucial in the context of cosmic shear studies.

Observations of galaxy-scale strong gravitational lensing systems enable unique tests of departures from general relativity at the kilo- to megaparsec scale. In this work, the gravitational slip parameter \ensuremathγ_PN, measuring the amplitude of a hypothetical fifth force, is constrained using 130 elliptical galaxy lens systems. We implement a lens model with a power-law total mass density and a deprojected De Vaucouleurs luminosity density, favored over a power-law luminosity density. To break the degeneracy between the lens velocity anisotropy \ensuremathβ and the gravitational slip, we introduce a new prior on the velocity anisotropy based on recent dynamical data. For a constant gravitational slip, we find \ensuremathγ_PN=0.9 0_-0.14^+0.18 in agreement with general relativity at the 68% confidence level. Introducing a Compton wavelength \ensuremathλ_g, effectively screening the fifth force at small and large scales, the best fit is obtained for \ensuremathλ_g\ensuremath∼0.2 Mpc and \ensuremathγ_PN=0.7 7_-0.14^+0.25. A local minimum is found at \ensuremathλ_g\ensuremath∼100 Mpc and \ensuremathγ_PN=0.5 6_-0.35^0.45. We conclude that there is no evidence in the data for a significant departure from general relativity and that using accurate assumptions and having good constraints on the lens galaxy model is key to ensure reliable constraints on the gravitational slip.