Lots! It has discovered about a million X-ray sources, about twice as many as previously known about. Most of these sources will be active galactic nuclei (supe

Lots! It has discovered about a million X-ray sources, about twice as many as previously known about. Most of these sources will be active galactic nuclei (supermassive black holes that are accreting material) in distant galaxies. There's still lots of work to do in characterising these objects though and understanding the population.