PRECESSION OF MAGNETICALLY DRIVEN WARPED DISKS AND LOW-FREQUENCY QUASI-PERIODIC OSCILLATIONS IN LOW-MASS X-RAY BINARIES

摘要

An accretion disk around a rotating magnetized star is subjected to magnetic torques that induce disk warping and precession. These torques arise generically from interactions between the stellar field and 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 make 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 rate M) 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). As M 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 large M. Because of this, and because the magnetically driven precession is retrograde (opposite to the Lense-Thirring precession) and depends strongly on M, the mode frequency can have a nonmonotonic 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 that are otherwise difficult to explain, such as the flattening/turnover in the LFQPO frequency-M correlation or in the LFQPO frequency-kHz QPO frequency correlation (e.g., as seen clearly in GX 17 + 2).