Of NBOs and kHz QPOs: a low-frequency modulation in resonant oscillations of relativistic accretion disks

摘要

The origin of quasi periodic modulations of flux in the kilohertz range (kHz QPOs), observed in low-mass X-ray binaries, is usually assumed to be physically distinct from that of the "normal branch oscillations" (NBOs) in the Z-sources. We show that a low-frequency modulation of the kHz QPOs is a natural consequence of the nonlinear relativistic resonance suggested previously to explain the properties of the high-frequency twin peaks. The theoretical results discussed here are reminiscent of the 6 Hz variations of frequency and amplitude of the kHz QPOs reported by Yu, van der Klis, and Jonker (2001, ApJ, 559, L29).