Precession of Magnetically Driven Warped Disks and Low‐Frequency Quasi‐periodic Oscillations in Low‐Mass X‐Ray Binaries

Abstract

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).

Evolution of the temporal and the spectral properties in 2010 and 2011 outbursts of H 1743-322

“…LFQPOs are reported extensively in the literature, although there is some uncertainty about the origin of these QPOs. So far, many models are introduced to explain the origin of this important temporal feature of BHCs, such as trapped oscillations and disko-seismology (Kato & Manmoto, 2000), oscillations of warped disks (Shirakawa & Lai, 2002), accretion-ejection instability at the inner radius of the Keplerian disk (Rodriguez et al, 2002), global disk oscillations (Titarchuk & Osherovich, 2000), and perturbations inside a Keplerian disk (Trudolyubov et al, 1999), propagating mass accretion rate fluctuations in hotter inner disk flow (Ingram & Done, 2011), and oscillations from a transition layer in between the disk and hot Comptonized flow (Stiele et al, 2013). However, none of these models attempt to explain long duration continuous observations and the evolutions of QPOs during the outburst phases of transient BHCs.…”

X-Ray and Optical Observations of a 0535+26

“…In their model, the coupling between the neutron star and the accretion disk occurs in a broad transition zone located between the flow inside the neutron star magnetosphere and the unperturbed disk flow. Another model, the magnetic disk precession model (Shirakawa & Lai 2002), attributes the mHz QPO in X-ray pulsars to warping/precession modes induced by magnetic torques near the inner edge of the accretion disk. We note that this model has only been applied to weakly magnetized neutron stars in low-mass X-ray binaries.…”

Forced oscillations in magnetized accretion disks and QPOs

“…It can, therefore, account for the low frequency QPOs (LFQPOs), as does the usual Lense-Thirring precession . Magnetic precession, combined with the relativistic dragging effect, was applied to weakly magnetized neutron stars to explain these LFQPOS in LMXBs, (Shirakawa & Lai 2002a). The study of the global magnetic warping/precession process for strongly magnetized neutron stars can also explain the millihertz QPOs in some systems, see for example Shirakawa & Lai (2002b).…”