Effects of the refractive index of the X-ray corona on the emission lines in AGNs

• Main Authors: Chainakun, P. (Author), Watcharangkool, A. (Author), Young, A.J (Author)
• Format: Article
• Language: English
• Published: Oxford University Press 2022
• Subjects: Accretion discs; accretion, accretion discs; Accretion: accretion disks; black hole physics; Black Hole physics; Black holes; Blue wings; Characteristic emission; Electromagnetic wave emission; Emission lines; Empty space; Galaxies; Galaxies active; galaxies: active; Geometrical optics; Gravitation; Light; Refractive index; Stars; X-ray reflections; X-rays: galaxies; X-rays: Galaxies
• Online Access: View Fulltext in Publisher

 X-ray reflection from an accretion disc produces characteristic emission lines allowing us to probe the innermost regions in active galactic nuclei. We investigate these emission lines under a framework of Riemannian geometrical optics where the corona has a refractive index of n ≠ 1. The empty space outside is a vacuum with n = 1. The Kerr metric is modified to trace the light rays that are bent due to not only the gravity of the black hole but also the effects of coronal plasma dependent on n. The choice of n alters the null geodesics, producing the effect which is analogous to the light deflection. For the corona with n > 1, the disc on the far side within the corona covers a larger area on the observer's sky, enhancing the blue wing of the line and producing more flux difference between the blue peak and extended red tail. The inverse effects are seen when n < 1. Moreover, the corona with n > 1 and n < 1 could induce extra shifts in the blue wing (Δgmax) to higher and lower energy, respectively. These effects are more prominent when the inclination angle is 60° and the corona extends to 5rg. To obtain the deviation of the line shift of Δgmax 0.01, the difference between the refractive index of the corona and that of the empty space must be Δ n ≳ 0.5 per cent. Finally, the lensing corona can influence the arrival time of photons that may affect the observed variability of these emission lines. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.