Abstract

The brightest black hole systems we can study are outbursting X-ray binaries in our galaxy. At the peaks of their outbursts, these systems show an array of phenomena that are linked to strong X-ray spectral evolution in the innermost accretion flow, such as mysterious X-ray quasi-periodic oscillations, which may be signatures of relativistic frame-dragging of material close to the black hole. X-ray observations are key to understanding these phenomena, but for many years these studies have been limited by the technical challenge of observing X-ray sources at high count rates, which can overwhelm detectors and significantly degrade the time and spectral-resolution of our instruments. The Neutron star Interior Composition ExploreR (NICER), installed on the International Space Station in 2017, is a new type of X-ray observatory which is capable of observing very bright X-ray sources with high time resolution and good spectral resolution, to provide a remarkable new view of the brightest black holes. I will present some of the key results on black hole X-ray binaries obtained by NICER so far and show how they can transform our view of the structure and evolution of accretion flows close to black holes.