A presentation at the Netherlands Astronomy Conference 2016.
X-ray spectral-timing is a burgeoning field that seeks to investigate how matter behaves in strong gravitational fields. Observations suggest that different types of quasi-periodic oscillations (QPOs) are associated with different emitting-region geometries (e.g. disk-like or jet-like) in the innermost part of the X-ray binary, close to the neutron star or black hole. We developed a technique for phase-resolved spectroscopy of QPOs, and are applying it to a variety of low-frequency QPOs from low-mass X-ray binaries containing black holes or neutron stars. On the QPO time-scale, we find that the energy spectrum changes not only in normalization, but also in spectral shape. In analyzing a variety of signals we will quantify how the spectral shape changes as a function of QPO phase and look for systematic trends between different classes of sources. We can then use these trends to infer the origin of the QPO and its relation to emitting-region geometry in the strong gravity regime.