A Possible Origin of kHZ QPOs in Low-Mass X-ray Binaries.

A possible origin of kHz QPOs in low-mass X-ray binaries is proposed. Recent numerical MHD simulations of accretion disks with turbulent magnetic fields of MRI definitely show the presence of two-armed spiral structure in quasi-steady state of accretion disks. In such deformed disks, two-armed ($m=2$) c-mode ($n=1$) oscillations are excited by wave-wave resonant instability. Among these excited oscillations, the fundamental in the radial direction ($n_r=0$) will be the higher kHz QPO of a twin QPOs, and the first overtone ($n_r=1$) in the radial direction will be the lower kHz QPO of the twin. A possible cause of the twin high-frequency QPOs (HFQPOs) in BH X-ray binaries is also discussed.