Reverberation mapping

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Reverberation mapping is to-day a statistical method within astrophysics preferably used in connection with velocity analysis of gaseous discs around supermassive compact objects. The technique is used to measure the size of the broad emission-line region and mass of the alleged central black hole in active galactic nuclei. ^[1]

Direct Doppler measures of water masers surrounding the nucleus of nearby galaxies have revealed a very fast keplerian motion, only possible with a high concentration of matter in the center. Currently, the only known objects that can pack enough matter in such a small space are black holes, or things that will evolve into black holes within astrophysically short timescales. For active galaxies farther away, the width of broad spectral lines can be used to probe the gas orbiting near the event horizon. The technique of reverberation mapping uses variability of these lines to measure the mass, and perhaps the spin of the presumed supermassive black hole that powers the active galaxy's "engine".