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Relativistic disc timing
Relativistic accretion discs not only produce Relativistic disc spectra but they are compact and highly variable. The corona is responsible for producing X-ray emission above the accretion disc, which it does by inverse-Compton scattering optical and UV photons from the disc. These X-rays reach the observer at infinity and also illuminate the accretion disc, producing a back-scattered "reflection" spectrum. Due to the increased path length that light has to follow between the corona and the disc, as well as the relativistic effects from being so close to a black hole, there will be a time delay between changes in the continuum and corresponding changes from the disc. This echo results in observable time delays, known as reverberation.
Observing thees time delays is difficult, and this usually needs to be done in Fourier space rather than by looking at individual bright flares. The first robust detection of this time delay was by Fabian et al. (2009) in observations of the Narrow Line Seyfert 1 (NLS1) galaxy 1H0707-495.
- "X-ray reverberation around accreting black holes" by Uttley at al. (2023)
- Fabian et al. (2009), "Broad line emission from iron K- and L-shell transitions in the active galaxy 1H0707-495"