Echoes from the distant Universe: X-ray reverberation mapping of gravitationally lensed quasars

Observing the reflection and reverberation of X-rays off the inner regions of the accretion disks around supermassive black holes gives us a unique view of the extreme environment just outside the event horizon. We can measure the spin of the black hole (an important quantity for understanding supermassive black hole growth) as well as the structure of the inner accretion flow and the X-ray emitting corona. X-ray reverberation measurements, however, have mostly been limited to a small subset of the supermassive black hole population; radio-quiet Seyfert galaxies in the local Universe.

Using new statistical techniques, we are now able to extend X-ray reverberation to a larger population of black holes. In particular, using a novel technique based on Gaussian processes, we are able to measure the time delays from lower signal to noise light curves from higher-redshift quasars that are magnified by gravitational lensing, as variations in the X-ray luminosity echo off the inner accretion disk. X-ray reverberation mapping of high redshift quasars enables us to more accurately measure the spin distribution supermassive black holes across a broader range of redshifts, to understand their growth, and to understand the accretion process onto the supermassive black holes in quasars that are still rapidly growing and influencing the development of their host galaxies via AGN feedback.