Probing General Relativity and the Extreme Environments around Black Holes during X-ray Flares

NuSTAR 2022: 10 years of the high-energy Universe in focus, Cagliari, Italy (21/06/2022)

Observing the reflection and reverberation of X-rays off the accretion disks around supermassive black holes gives us a unique insight into the extreme environment just outside the event horizon. We can measure structure of the accretion flow and the corona that produces the X-ray continuum emission, and observe the effects of General Relativity in the strong gravitational field around the black hole.

NuSTAR observations of bright X-ray flares and how they echo off the accretion disc have led to unprecedented discoveries. They show how the corona is evolving, giving rise to the extreme variability we observe. We are discovering how, during bright X-ray flares, the temperature of the corona drops as it is launched away from the disk, giving us important clues as to how the corona is powered by the accretion flow.

In addition to teaching us about the corona, the echoes of the flare from the accretion disk allow us to measure light bending around the black hole and test key predictions of General Relativity. Observations of these bright X-ray flares reveal the first observational evidence for the reverberation of the X-ray flare from material that should classically be hidden behind the shadow of the black hole, re-emerging as the reflected X-rays are bent around the black hole into our line of sight.