2026ApJ...999..215S

We perform milliarcsecond X-ray astrometry of the quadruply lensed radio-quiet quasar GraL J065904.1+162909 (J0659). This z_s = 3.083 quasar is lensed into four images and was discovered with the second Data Release of the Gaia Space Observatory (Gaia DR2). Our J0659 study exploits strong gravitational lenses as high-resolution telescopes. This technique shows promise to elucidate the origin of optical and X-ray emission in distant lensed quasars at spatial scales beyond the reach of current instruments. In our study, we use Gaia DR3 and Hubble Space Telescope observations of J0659 to infer a mass model for the deflector. Our model reproduces the Gaia DR3 quasar lensed image positions to one milliarcsecond and determines the position of the optical source in J0659 to within this precision. Next, we analyze Chandra observations of J0659 and conduct a Bayesian test evaluating whether the X-ray emission region coincides with the optical source. We then constrain the origin of the X-ray emission to within a $0\mathop{.}\limits^{^{\prime\prime} }020$ × $0\mathop{.}\limits^{^{\prime\prime} }010$ ellipse centered $0\mathop{.}\limits^{^{\prime\prime} }014$ away from the optical source at the 1σ level. We demonstrate that our approach can be extended to pinpoint the distinct locations of the soft and hard X-ray emission regions in lensed quasars. We discuss the potential of upcoming broadband and spectrally resolved X-ray astrometric studies to probe complex quasar morphology and multiplicity of active galactic nuclei at subkiloparsec scales otherwise inaccessible at high redshifts.