Magnetically Driven Precession of Warped Disks and Milli-Hertz Variabilities in Accreting X-Ray Pulsars

The inner region of the accretion disk around a magnetized neutron star is subjected to magnetic torques that induce warping and precession of the disk. These torques arise from interactions between the stellar field and the induced electric currents in the disk. We carry out a global analysis of warping/precession modes in a viscous accretion disk, and show that under a wide range of conditions typical of accreting X-ray pulsars, the magnetic warping torque can overcome viscous damping and make the mode grow. The warping/precession modes are concentrated near the inner edge of the disk (at the magnetosphere-disk boundary), and can give rise to variabilities or quasi-periodic oscillations (QPOs) in the X-ray/UV/optical fluxes from X-ray pulsars. We examine the observed properties of mHz QPOs in several systems (such as 4U 1626-67), and suggest that some hitherto unexplained QPOs are the results of magnetically driven disk warping/precession.