Coronal Self-Oscillations: A Novel Framework for Understanding Low-Frequency Quasi-Periodic Oscillations

Low-frequency quasi-periodic oscillations (LFQPOs) frequently manifest in the X-ray flux of black hole binaries, appearing as coherent peaks in the power spectrum with centroid frequencies spanning 0.01-10 Hz. Despite their prevalence and measurable attributes, the exact origin of LFQPOs remains elusive, defying comprehensive explanations from existing models. This work proposes a novel framework wherein LFQPOs originate in the corona due to self-oscillations. Self-oscillations can generate and maintain a periodic motion by a power source that lacks periodicity. As a result, unlike in the scenario of a resonant case, there is no need to find a periodic driver. These self-oscillations associated with the corona can create the X-ray modulation detected as a QPO. Our model characterizes these QPOs as thermoacoustic self-oscillations, in which the acoustic oscillation of the corona is maintained by positive feedback with the rate at which the corona is cooled by inverse Compton scattering of soft photons coming from the accretion disk around the black hole. The mechanical oscillation of the corona is maintained against its internal viscous damping because cooling is most effective when the corona is at the lowest temperature and largest volume within the QPO cycle, analogous to the classical “kappa mechanism” explaining pulsations in variable stars. In the case of the QPOs, we propose that the feedback arises because pair production lowers the temperature of the plasma in the corona (due to equipartition) while increasing the plasma’s optical depth (making the cooling by inverse Compton scattering more efficient). This thermodynamic perspective not only provides a mechanism for LFQPOs but also offers insights into the elusive physical properties of the corona itself, promising a more comprehensive understanding of these intriguing astrophysical phenomena.