A Case of the Lensed AGN ULAS J081621.49+213442.65 at <i>z</i> ∼ 7.45 Confused with an Asteroid

Abstract S. E. I. Bosman et al. proposed that the United Kingdom Infrared Telescope Infrared Deep Sky Survey Large Area Survey (UKIDSS LAS) detections of the high-redshift active galactic nucleus ULAS J081621.49+213442.65 in the 2008 data were caused by the transit of a moving object, rather than by ULAS J081621.49+213442.65 itself. Here, we have applied the optimal image subtraction method to the UKIDSS LAS J -band images from 2008, in which ULAS J081621.49+213442.65 was first detected. The resulting difference image showed that the transient of S. E. I. Bosman et al. has a previously unreported nearest counterpart located ∼24″ away from ULAS J081621.49+213442.65. The proposed identification of the source with a moving object is thus ruled out. ULAS J081621.49+213442.65 is a gravitationally lensed system consisting of four lensed images with a maximum separation of ∼2″. Astrometric and photometric behaviors similar to those of the UKIDSS LAS data were independently observed in 2024 during significant brightening of the same lensed images of ULAS J081621.49+213442.65 by &gt;1 mag.