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

P Chainakun, A Watcharangkool, A J Young

Research output: Contribution to journalArticle (Academic Journal)peer-review

3 Citations (Scopus)

Abstract

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 Δn≳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.
Original languageEnglish
Pages (from-to)728-738
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Volume512
Issue number1
DOIs
Publication statusPublished - 10 Feb 2022

Bibliographical note

Funding Information:
We thank the re vie wer, Javier A. Garcia, for useful comments and suggestions that help clarify the manuscript. This w ork w as supported by (i) Suranaree University of Technology (SUT), (ii) Thailand Science Research and Innovation (TSRI), and (iii) National Science Research and Innovation Fund (NSRF), project no. 160355. AW acknowledges financial support from National Astronomical Research Institute of Thailand. The numerical calculations were car- ried out using the BLUECRYSTAL supercomputer of the Advanced Computing Research Centre, University of Bristol.

Publisher Copyright:
© 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.

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