New techniques for understanding rapid X-ray variability from compact objects

Compact objects like stellar-mass black holes and neutron stars are dense enough to significantly warp spacetime. By studying emission from very close to the compact object, we can decipher the effects of strong-gravity on physical processes, and test general relativity in the strong-field limit. There is a plethora of rapid sub-second variability in the X-ray light curves from compact objects in accreting low-mass X-ray binaries (LMXBs), and a growing toolbox of analysis techniques and algorithms to apply to such phenomena. This thesis investigates quasi-periodic oscillations (QPOs) and coherent X-ray burst oscillations. QPOs are a probe for studying physical processes in the inner regions of LMXBs, and X-ray burst oscillations are used to determine the masses and radii of neutron stars to constrain the neutron star equation of state. In Chapter 2, we present a novel spectral-timing technique to do phase-resolved spectroscopy of QPOs that tracks the variations of spectral parameters with QPO phase, and we apply it to a low-frequency QPO from the black hole GX 339-4. In Chapter 3, we use ray-tracing to simulate pulse profiles of thermonuclear burst oscillations from an accreting neutron star and fit these with an evolutionary optimization algorithm. In Chapter 4, we apply our phase-resolved spectroscopy technique to a lower kilohertz (kHz) QPO from the neutron star 4U 1608-52. Finally in Chapter 5, we carry out spectral-timing analysis of the low-frequency QPO seen by NICER in the new black hole transient MAXI J1535-571