The Rhythm of Accretion:time lags of the fast variability in X-ray binaries

Characterising fast variability in the emission of low-mass X-ray binaries (LMXBs) opens a window into understanding the true nature of the accretion process onto neutron stars and black holes. The short timescales over which changes in the X-ray emission of LMXBs occur associate this fast variability, better known as quasi-periodic oscillations (QPOs), with the innermost regions of the system – closest to the compact object – where matter is under the influence of unique extreme conditions. Simultaneously studying the properties of the QPOs and the LMXB can shed light on the origin of the variability and its relation with the accretion phenomena. Through a systematic study of a sample of neutron-star LMXBs, we found that the properties of the high-frequency QPOs observed in these systems depend on the luminosity of the source. This result suggests that coupled oscillations of the physical components in the system can be responsible for the variability we observe. We confirmed these conclusions by studying the high-frequency QPOs in the neutron-star system XTE J1701-462, an LMXB that drastically changed its luminosity during one accretion cycle or outburst. To identify the geometrical configuration responsible for the QPOs in LMXBs, we used a theoretical model that simultaneously characterises the properties of the QPOs and the energy spectrum of the source. Using this model, we found that the low-frequency QPOs in the black-hole LMXB GX 339-4 can be explained by two physically-connected comptonizing regions or coronae that interact with the accretion disc via a feedback loop of X-ray photons