Milliarcsecond X-Ray Positions and X-Ray Varstrometry for the Strongly Lensed Active Galactic Nucleus HE 0435-1223

Abstract Active galactic nuclei (AGN) are some of the most powerful objects in the Universe. For this reason, they can be observed up to high redshifts ( z ), giving valuable insights into the evolution of our Universe. However, high- z AGN are too distant to be spatially resolved with current or upcoming X-ray facilities. In this paper, we show how we can exploit gravitationally lensed AGN to significantly increase spatial resolution even at high z . We combine astrometric data from Gaia DR3 with imaging from the Chandra X-ray Observatory of the quadruply lensed quasar HE 0435-1223 to measure for the first time possible offsets between the optical and the X-ray emissions. We measure the X-ray source position for HE 0435-1223 within a 1 σ quasi-elliptical region of 0.5 × 1.3 milliarcsecond (mas), about 150 pc 2 at the redshift of the source ( z = 1.689). We find evidence for the X-ray emission being offset by a projected 3 mas from the Gaia (optical) emission. The positional offset is most likely associated with a portion of the X-ray emission arising from an X-ray jet or outflow. We also discuss how this method can be used to indicate the presence of a binary/offset AGN system.