Astrophys. J. Lett. 895, L38 (2020)

Einstein rings are perhaps gravity’s most spectacular manifestations, with light from a background source being bent as it passes close to a foreground massive object. Daniel Stern and Dominic Walton take a stroll down memory lane to revisit the first-ever Einstein ring, discovered in 1988 by Jacqueline Hewitt (pictured, at radio frequencies). By analysing archival optical and X-ray data, the authors make a precise measurement of the lensed galaxy’s redshift and reveal it to be an obscured quasar.

Credit: Springer Nature Ltd

The (re)analysis of optical spectra from the Keck telescope reveals two narrow emission lines, which the authors identify as [C iii] and He ii. These high-ionization narrow lines are typical for obscured quasars, and pinpoint the redshift of this source to z = 1.849 ± 0.002. Stern and Walton also analyse archival X-ray observations from Chandra that clearly reveal two X-ray images of the lensed quasar. Multiple images were originally seen in radio (A1+A2 and B in the figure). By modelling its X-ray spectrum, the authors calculate a photon index (Γ = \(1.7_{-0.3}^{ + 0.4}\)) and column density (ΝΗ = \(3.0_{-1.5}^{ + 1.7} \times 10^{22}\) cm–2) that are typical of this kind of source.

With an accurate distance at hand, the authors note that the lensed galaxy appears more luminous in the X-rays than expected from its mid-infrared luminosity. Due to lensing magnification, Stern and Walton propose this X-ray excess as a way to identify candidate lensed active galaxies in wide-area X-ray surveys like the one currently performed by eROSITA. The derived redshift opens the door to a multitude of possibilities for following up this rare lensed obscured quasar and for precise modelling of the lens system as a whole.