Abstract

We present results regarding the disc response functions and the corresponding time lags assuming a standard Novikov–Thorne accretion disc illuminated by a point-like X-ray source. We took into account all relativistic effects in the light propagation from the X-ray source to the disc and then to the observer, and we computed the disc reflection, accounting for its radial ionization profile. Our results suggest that the thermal reverberation effects should be stronger in sources with large X-ray source height and low accretion rate. We found that time lags increase with height and accretion rate. The amplitude of the observed time lags as a function of wavelength (in NGC 5548) is consistent with the model predictions. It is not necessary for the disc to be too hot, it may be that the X-ray source is located further from the disc.

Highlights

  • Active galactic nuclei (AGN) are thought to be powered by accretion of matter onto a supermassive black hole (BH, with a mass MBH ∼106−9 M ) in the form of an optically-thick, geometrically-thin disc [1,2]

  • If the X-ray source is small in size, if it is located above the BH at height h, and if the X-rays are emitted isotropically, they will irradiate the accretion disc

  • We investigated the effects of the X-ray source height and the accretion rate on the disc response functions and the lag spectra

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Summary

Introduction

Active galactic nuclei (AGN) are thought to be powered by accretion of matter onto a supermassive black hole (BH, with a mass MBH ∼106−9 M ) in the form of an optically-thick, geometrically-thin disc [1,2]. If the X-ray source is small in size, if it is located above the BH at height h (i.e., the ‘lamp-post’ geometry), and if the X-rays are emitted isotropically, they will irradiate the accretion disc. If the X-rays are variable, we expect the part of the UV/optical emission which is due to X-ray absorption to vary with delays increasing with wavelength. We present results from a recent study [3] of the time lags vs wavelength relation (hereafter “lag-spectrum”) in the context of the lamp-post geometry. We investigated the effects of the X-ray source height and the accretion rate on the disc response functions and the lag spectra. UV/optical lags in this source, as long as the X-ray source height is larger than 40 rg

Model Setup
The Disc Response Functions
The Time Lags in NGC 5548
Conclusions

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