The central engine of an Active Galactic Nucleus (AGN) is a supermassive black hole, which accretes matter from its surroundings. This accretion process forms a disk of matter around the black hole, known as the accretion disk. As the matter spirals inward towards the black hole, it loses gravitational potential energy, which is converted into kinetic and thermal energy. This energy conversion process heats up the innermost regions of the disk to extremely high temperatures, causing them to emit X-rays. The X-ray emission from the accretion disk is primarily due to two processes: thermal emission and non-thermal emission. Thermal emission, also known as blackbody radiation, occurs when the hot gas in the inner accretion disk emits X-rays directly. Non-thermal emission, on the other hand, occurs when high-energy particles in the disk collide with photons, boosting their energy up to the X-ray range. This process is known as inverse Compton scattering.
Many AGN exhibit a 'soft excess' at energies less than ~2 keV, which sits above what would be expected from the hard X-ray power-law. This excess emission is often fitted using a black body model, yielding best-fit temperatures in the range of 0.1-0.2 keV. However, the origin of the soft X-ray excess is not well understood and various models have been proposed, including disk reflection, a warm corona, and magnetic reconnection.
Above the accretion disk is a region of even hotter, less dense gas known as the corona. The corona is thought to be responsible for a significant amount of the X-ray emission observed from AGN. The X-rays from the corona can irradiate the accretion disk, causing it to fluoresce and emit its own X-rays. This process is known as Compton reflection or coronal upscattering. In particular, the inverse Compton scattering of photons from the geometrically thin disk by hot electrons in the corona quickly cools the coronal gas. The reflection component in the X-ray spectra of AGN arises from the interaction of the X-ray photons with the accretion disk. This interaction results in the reflection of a fraction of the incident radiation, which forms a characteristic 'echo' in the X-ray light curve. The reflection component is crucial for understanding the geometry and dynamics of the corona and the disk.
The strong gravitational field of the black hole causes the light paths to bend and the X-ray frequencies to shift, leading to effects such as gravitational redshift and time dilation. These relativistic effects can distort the reverberation lags and the reflection spectrum, providing a probe of the black hole's mass and spin. In the context of AGN, 'lags' refer to the time delay between variations in different energy bands of the X-ray emission. At the lowest frequencies, hard lags are observed where the power-law dominated hard band lags the soft band. However, at higher frequencies, soft lags are observed. These lags are interpreted as due to reverberation from the accretion disk, with the reflection component responding to variability from the X-ray corona. The reverberation lags can be measured as a function of frequency using Fourier techniques, revealing the transfer function or impulse response of the system. The lag-frequency and lag-energy spectra can provide information about the geometry and dynamics of the corona and the disc.