NICER observation list: Target ID 559901

BH LMXBs show prominent low-frequency QPOs in their power-density spectra, with fractional amplitudes that increase with energy and complex lag spectra. The question about their physical origin remains unanswered. Quasi-simultaneous observations with NICER and NuSTAR in the soft and hard X-ray bands will allow us to probe the radiative mechanism that modulates the energy-dependent timing properties of these LF QPOs. This information is crucial to unveil the physical mechanism that produces these QPOs, both dynamically, either connected to Lense-Thirring Precession or through instabilities in the hot accretion flow, and radiatively, via Compton amplification in the corona where the observed spectral-timing properties are imprinted onto the X-ray emission.