Reflection from the strong gravity regime in a lensed quasar at redshift z = 0.658

Abstract

The co-evolution of a supermassive black hole with its host galaxy[1] through cosmic time is encoded in its spin[2, 3, 4]. At z > 2, supermassive black holes are thought to grow mostly by merger-driven accretion leading to high spin. However, it is unknown whether below z ∼ 1 these black holes continue to grow via coherent accretion or in a chaotic manner [5], though clear differences are predicted [3,4] in their spin evolution. An established method[6] to measure the spin of black holes is via the study of relativistic reflection features [7] from the inner accretion disk. Owing to their greater distances, there has hitherto been no significant detection of relativistic reflection features in a moderateredshift quasar. Here, we use archival data together with a new, deep observation of a gravitationally-lensed quasar at z = 0.658 to rigorously detect and study reflection in this moderate-redshift quasar. The level of relativistic distortion present in this reflection spectrum enables us to constrain the emission to originate within 3 gravitational radii from the black hole, implying a spin parameter a = 0.87 +0.08 −0.15 at the 3σ level of confidence and a > 0.66 at the 5σ level. The high spin found here is indicative of growth via coherent accretion for this black hole, and suggests that black hole growth between 0.5 z 1 occurs principally by coherent rather than chaotic accretion episodes. 1When optically-thick material, e.g. an accretion disc, is irradiated by hard X-rays, some of the flux is reprocessed into an additional 'reflected' emission component, which contains both continuum emission and atomic features. The most prominent signature of reflection from the inner accretion disc is typically the relativistic iron Kα line (6.4-6.97 keV; rest frame) [8] and the Compton reflection hump [7] often peaking at 20 − 30 keV (rest-frame). However, the deep gravitational potential and strong Doppler shifts associated with regions around black holes will also cause the forest of soft X-ray emission lines in the ∼ 0.7 − 2.0 keV range to be blended into a smooth emission feature, providing a natural explanation for the "soft-excess" observed in the X-ray spectra of nearby active galactic nuclei (AGNs) [9,10]. Indeed, both the iron line and the soft excess can be used to provide insight into the nature of the central black hole and to measure its spin [10]. Prior studies have revealed the presence of a soft-excess in 90% of quasars at[11, 12] z 1.7, and broad Fe-lines are also seen in 25% of these objects [11,13], suggesting that reflection is also prevalent in these distant AGNs. However, due to the inadequate S/N resulting from their greater distances, the X-ray spectra of these quasars were necessarily modeled using simple phenomenological parameterizations [12,14].Gravitational lensing offers a rare opportunity to study the innermost relativistic region in distant quasars [15,16], by acting as a natural telescope and magnifying the light from these sources. Quasars located between 0.5 z 1 are considerably ...

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