Discovery of Drifting High-frequency QPOs in Global Simulations of Magnetic Boundary Layers

Abstract

We report on the numerical discovery of quasi-periodic oscillations (QPOs) associated with accretion through a non-axisymmetric magnetic boundary layer in the unstable regime, when two ordered equatorial streams form and rotate synchronously at approximately the angular velocity of the inner disk The streams hit the star's surface producing hot spots. Rotation of the spots leads to high-frequency QPOs. We performed a number of simulation runs for different magnetospheric sizes from small to tiny, and observed a definite correlation between the inner disk radius and the QPO frequency: the frequency is higher when the magnetosphere is smaller. In the stable regime a small magnetosphere forms and accretion through the usual funnel streams is observed, and the frequency of the star is expected to dominate the lightcurve. We performed exploratory investigations of the case in which the magnetosphere becomes negligibly small and the disk interacts with the star through an equatorial belt. We also performed investigation of somewhat larger magnetospheres where one or two ordered tongues may dominate over other chaotic tongues. In application to millisecond pulsars we obtain QPO frequencies in the range of 350 Hz to 990 Hz for one spot. The frequency associated with rotation of one spot may dominate if spots are not identical or antipodal. If the spots are similar and antipodal then the frequencies are twice as high. We show that variation of the accretion rate leads to drift of the QPO peak.

13 pages, 8 figures, accepted by MNRAS, v4: final version after significant revision, added new figures, Table 2. See sample animations as: http://www.astro.cornell.edu/~romanova/qpo.htm

References in corpus (12)

• Accretion to Magnetized Stars through the Rayleigh-Taylor Instability: Global Three-Dimensional Simulations
• Accretion funnels onto weakly magnetized young stars
• Unstable Disk Accretion to Magnetized Stars: First Global 3D MHD Simulations
• Discovery of coherent millisecond X-ray pulsations in Aql X-1
• Intermittent millisecond X-ray pulsations from the neutron-star X-ray transient SAX J1748.9-2021 in the globular cluster NGC 6440
• Accretion to Stars with Non-dipole Magnetic Fields
• Three-dimensional Simulations of Accretion to Stars with Complex Magnetic Fields
• Screening of the Magnetic Field of Disk Accreting Stars
• Wave Excitation in Disks Around Rotating Magnetic Stars
• Quasi-periodic oscillations as global hydrodynamic modes in the boundary layers of viscous accretion disks
• Three Disk Oscillation Modes of Rotating Magnetized Neutron Stars
• 2D and 3D MHD Simulations of Disk Accretion by Rotating Magnetized Stars: Search for Variability

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• Analysis of variability of TW Hya as observed by MOST and ASAS in 2009
• Stable and unstable accretion in the classical T Tauri stars IM Lup and RU Lup as observed by MOST
• A stable quasi-periodic 4.18 d oscillation and mysterious occultations in the 2011 MOST light curve of TWHya
• The Magneto Hydro Dynamical Model of KHz Quasi Periodic Oscillations in Neutron Star Low Mass X-ray Binaries (II)
• Phase lags of quasi-periodic oscillations across source states in the low-mass X-ray binary 4U 1636-53
• Modeling of Disk-Star Interaction: Different Regimes of Accretion and Variability
• Observing Rayleigh-Taylor stable and unstable accretion through a Kalman filter analysis of X-ray pulsars in the Small Magellanic Cloud
• Oscillations of the Boundary Layer and High-frequency QPOs