QPOs in Compact Sources as a Nonlinear Hydrodynamical Resonance: Determining Spin of Compact Objects

Abstract

The origin of wide varieties of quasiperiodic oscillations (QPOs) observed in compact sources is still not well established. Its frequencies range from millihertz to kilohertz spanning all compact objects. Are different QPOs, with different frequencies, originating from different physics? We propose that the emergence of QPOs is the result of nonlinear resonance of fundamental modes present in accretion disks forced by external modes including that of the spin of the underlying compact object. Depending on the properties of accreting flow, e.g., its velocity and gradient, resonances (and any mode-locking) take place at different frequencies, exhibiting low- to high-frequency QPOs. We explicitly demonstrate the origin of higher-frequency QPOs for black holes and neutron stars by a unified model and outline how the same physics could be responsible for producing lower-frequency QPOs. The model also predicts the spin of black holes, and constrains the radius of neutron stars and the mass of both.