Advances in phase resolving quasi-periodic oscillations with NICER

The X-ray emission of accreting, stellar-mass black holes is variable on a wide range of timescales and often shows quasi-periodic oscillations, or QPOs. Discovered decades ago, the origin of these QPO remains an open question to this day. In the most promising QPO model, the inner accretion flow precesses due to relativistic frame dragging. In addition to X-ray variability, such geometric changes should cause distinct spectral variations with the QPO phase. Phase-resolving the QPO for spectral analysis is therefore an important step towards understanding its physical origin. In this talk, I will discuss how the recently deployed NICER mission, with its CCD-like energy resolution and soft response, has impacted such QPO studies. After introducing different phase-resolving methods, I will compare their results and the improvements brought on by NICER, by presenting a case-study of the QPO in NICER observations the recently discovered accreting black hole MAXI J1535-571.