Tracing activity in distant supermassive black holes with X-rays

Supermassive black holes at the centres of galaxies have long been thought to be the engines of quasars, which emit more energy than any other sources in the Universe. In the local Universe, dormant supermassive black holes have been detected through the motions of stars and gas near the galactic centres. In the distant Universe, high energy X-ray observations are now revealing the accretion of matter onto supermassive black holes, even when the black holes are highly obscured by gas and dust. Great advances are being made in obtaining a cosmic census of supermassive black holes. The duration, times, and mass inflow rates to these black holes are being traced via multiwavelength follow-up observations with ground-based telescopes and a time history of the accretion is thereby being reconstructed.