Milliarcsecond X-Ray Astrometry to Resolve Inner Regions of AGN at z > 1 Using Gravitational Lensing

Abstract

We report the localization of the X-ray emission from two strongly lensed AGN, CLASS B0712+472 ( z = 1.34) and CLASS B1608+656 ( z = 1.394). We obtain milliarcsecond X-ray astrometry by developing a novel method that combines parametric lens modeling with a Bayesian analysis. We spatially locate the X-ray sources in CLASS B0712+472 and CLASS B1608+656 within 11 mas and 9 mas from the radio source, respectively. For CLASS B0712+472, we find that the X-ray emission is cospatial with the radio and optical emission. On the other hand, for CLASS B1608+656, the X-ray emission is cospatial with radio but displaced with respect to the optical emission at the 1 σ level, which positions this source as an offset AGN candidate. This high astrometric precision improves on the limitations of existing X-ray instruments by two orders of magnitude. The demonstrated method opens a path to search for offset and binary AGN at z > 1, and to directly test supermassive black hole formation models in a redshift range that has been mostly underconstrained to date.