The Central Engine: Observations

43rd COSPAR Scientific Assembly, Sydney, Australia and online (31/01/2021) - invited

The accretion of matter onto supermassive black holes powers some of the most luminous objects we see in the Universe; active galactic nuclei (AGN). Intense emission is produced across the electromagnetic spectrum; optical and UV emission from the accretion disc and X-rays produced by a corona of accelerated particles close to the black hole, along with powerful winds and vast jets travelling at close to the speed of light that are seen from many black holes.

Observations of supermassive black holes across the electromagnetic spectrum reveal a wealth of information about the central engine. From the reflection and reverberation of X-rays o˙ the innermost regions the accretion disc, a three-dimensional picture is starting to emerge of the extreme environments around black holes. Reverberation reveals the structure of the disc and the corona, the e˙ects of strong light bending in the gravitational field around the black hole, and how the launching sites of jets may be connected to the corona and inner disc. During flares and other transient events, we see how the corona evolves, giving rise to extreme variations in luminosity and how the corona can be ejected in failed jet-launching events, while providing a unique opportunity to observe the e˙ects of General Relativity.

Observations of the central engines in AGN yield important insight into the small-scale processes close to the event horizon that enable black holes to power some of the most extreme objects in the Universe, launch vast jets at close to the speed of light and play their important feedback role in the formation of structure in the Universe.