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.
An accretion disk around a rotating magnetized star is subjected to magnetic torques which induce disk warping and precession. These torques arise generically from interactions between the stellar field and the induced surface currents on the disk. Applying these new effects to weakly magnetized (B ∼ 10 7 -10 9 G) neutron stars in low-mass X-ray binaries, we study the global hydrodynamical warping/precession modes of the disk under the combined influences of relativistic frame dragging, classical precession due to the oblateness of the neutron star, and the magnetic torques. Under quite general conditions, the magnetic warping torque can overcome the "Bardeen-Petterson" viscous damping and makes the modes grow. The modes are confined to the inner region of the disk, and have frequencies equal to 0.3 − 0.95 (depending on the mass accretion rateṀ ) times the sum of the Lense-Thirring frequency, the classical precession frequency, and the magnetically driven precession frequency evaluated at the inner disk radius r in . AsṀ increases, the mode frequency is reduced relative to the total precession frequency at r in since the mode becomes less concentrated around r in due to the increasing viscous stress associated with the largeṀ . Because of this, and because the magnetically driven precession is retrograde (opposite to the Lense-Thirring precession) and depends strongly oṅ M , the mode frequency can have a non-monotonic dependence on the mass accretion rate. This may account for several observed features of low-frequency (10-60 Hz) quasi-periodic oscillations (LFQPOs) in low-mass X-ray binaries which are otherwise difficult to explain, such as the flattening/turnover in the LFQPO frequency -Ṁ correlation or in the LFQPO frequency -kHz QPO frequency correlation (e.g., as seen clearly in GX 17+2).
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