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X-ray reverberation echoes are assumed to be produced in the stronglydistorted spacetime around accreting supermassive black holes. This signalallows us to spatially map the geometry of the inner accretion flow - a regionwhich cannot yet be spatially resolved by any telescope - and provides a directmeasure of the black hole mass and spin. The reverberation timescale is set bythe light travel path between the direct emission from a hot X-ray corona andthe reprocessed emission from the inner edge of the accretion disc. However,there is an inherent degeneracy in the reverberation signal between black holemass, inner disc radius and height of the illuminating corona above the disc.Here, we use a long X-ray observation of the highly-variable active galaxy,IRAS 13224-3809, to track the reverberation signal as the system evolves ontimescales of a day. With the inclusion of all the relativistic effects,modelling reveals that the height of the X-ray corona increases with increasingluminosity, providing a dynamic view of the inner accretion region. Thissimultaneous modelling allows us to break the inherent degeneracies and obtainan independent timing-based estimate for the mass and spin of the black hole.The uncertainty on black hole mass is comparable to the leading opticalreverberation method, making X-ray reverberation a powerful technique,particularly for sources with low optical variability.

A dynamic black hole corona in an active galaxy through X-ray reverberation mapping