Fourier spectral-timing techniques for the study of accreting black holes

The X-ray signal from active galactic nuclei and black hole (BH) X-ray binaries is highly variable on a range of timescales. This variability can be exploited to map the region of interest close to the BH, which is far too small to directly image for all but two BHs in the Universe. Spectral-timing techniques provide causal information by combining timing and spectral information. I present a brief review of such techniques, focusing on two examples: X-ray reverberation mapping and phase-resolved spectroscopy of low-frequency quasi-periodic oscillations (LF QPOs). The former provides a means to diagnose the accretion geometry and measure parameters such as BH mass, and the latter gives perhaps the best constraints we currently have as to the enigmatic LF QPO mechanism.