Determining the spatial origin of X-ray and optical emission in the $z=3.1$ strongly lensed radio-quiet quasar GraL J065904.1+162909 to hundreds of parsecs

Abstract

We perform milliarcsecond X-ray astrometry of the quadruply lensed radio-quiet quasar GraL J065904.1+162909 (J0659). This $z=3.083$ 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 $HST$ 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 $0{.}^{\prime \prime }020\times 0{.}^{\prime \prime }010$ ellipse centered $0{.}^{\prime \prime }014$ away from the optical source at the $1\sigma$ level. We demonstrate that our approach can be extended to pinpoint the distinct origins 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 AGN multiplicity at sub-kiloparsec scales otherwise inaccessible at high redshifts.

16 pages, 7 figures, 4 tables. Accepted for publication in ApJ in January 2026. Published in March 2026 (ApJ, Volume 999, Issue 2). Comments welcome!