X-ray reverberation in AGN
The animations below show the toy model to illustrate the X-ray reverberation in AGN. The basic idea is that the emission from the primary source (patch of hot corona) is reprocessed and reflected from the disc with a certain time delay given by the geometry of the system. The response of the disc is characterized by the transfer function which, in fact, is the light curve of the reflection echo due to a short flare. The transfer function tells us how the disc responds to the illuminating radiation in different frequency and energy domains. Currently planned X-ray mission Athena will be able to measure such a transfer function in AGN.
The response of the accretion disc consists of the reflected continuum emission as well as the fluorescent lines. The reverberation in neutral Kalpha line around extremely rotating Kerr black hole and non-rotating Schwarzschild black hole is compared in this animation. The left panels show the development of the echo as the illumination advances across the disc. The outline of the black hole horizon is highlighted with the white colour. The larger white "circle" in the Schwarzschild case denotes the innermost stable circular orbit (ISCO) below which the matter is expected to fall down onto the black hole rather quickly and thus creating a hole in the accretion disc. The colour in these figures shows the energy of the intrinsically narrow spectral line at 6.4 keV which illustrates the relativistic shift of the photon energy due to strong gravity and fast motions near the black hole. Note that for highly rotating black holes two echos, outer and inner, are gradually established, the latter one due to the fact that it takes long for the light to escape from the strong gravity in close vicinity of the black hole. The middle panels represent the time dependence of the spectral response. Right panels depict the light curves (top figure) and evolving spectra (bottom figure).
The animations are computed for the isotropic on-axis point-like patch of the corona at the height of 3 GM/c2 above the black hole and for three inclinations of the system, 30°, 60° and 85° (measured from the axis). The timescale of the animation goes up to 30 GM/c3 that corresponds to a 15 second animation. Thus the speed of the movie is tuned for the black hole of 105 solar masses. The duration of the animation should be prolonged to 10 minutes for the Galactic supermassive black hole and to more than 4 hours for an AGN black hole of 108 solar masses.