Low-Frequency QPOs as Coronal Normal Modes

Low-frequency quasi-periodic oscillations (LFQPOs) are frequently present in the X-ray flux of black hole binaries. They appear in the power spectrum as coherent peaks. Their centroid frequencies are in the range of 0.01-10 Hz. Even though they have been identified in various sources and are readily measurable, the exact nature of their origin still eludes us. Thus far, no models have provided a comprehensive or satisfactory explanation for them. In this work, we propose that the origin of these low-frequency QPOs is related to the corona. Specifically, that the LFQPOs are coronal normal modes. These resonant modes associated with the corona can create the X-ray modulation detected as a QPO. This model can seamlessly incorporate the observed variation in QPO frequencies and reverberation lags as a change in the dimensions of the resonant region. Furthermore, here, the disappearance of the QPO can be interpreted as a break of the confined resonant conditions. Another instrumental consequence of this model is that it can shed light on the elusive physical properties of the corona itself.