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— Side view of the magnetic field and accretion disk in accreting neutron stars. The neutron star's strong gravity causes a very high velocity flow toward the magnetosphere. As a result, the magnetosphere is pushed inward in the disk plane but balloons outward in direction away from the disk plane. Some of the plasma may leave the disk and flow along the field lines.
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In this study we propose a new generic model for QPOs based on oscillation modes of neutron star magnetospheres. We argue that the interaction of the accretion disk with the magnetosphere can excite resonant shear Alfven waves in a region of enhanced density gradients. We demonstrate that depending on the distance of this enhanced density region fr...
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In this paper, we improve the previous work on the MHD Alfvén wave oscillation model for the neutron star (NS) kHz quasi-periodic oscillations (QPOs), and compare the model with the updated twin kHz QPO data. For the 17 NS X-ray sources with the simultaneously detected twin kHz QPO frequencies, the stellar mass M and radius R constraints are given by means of the derived parameter A in the model, which is associated with the averaged mass density of the star as 〈ρ 〉 = 3M /(4πR3) ≃ 2.4 × 1014 (A /0.7)2 g/cm3, and we also compare the M -R constraints with the stellar equations of state. Moreover, we also discuss the theoretical maximum kHz QPO frequency and maximum twin peak separation, and some expectations on SAX J1808.4–3658 are mentioned, such as its highest kHz QPO frequency ∼ 870 Hz, which is about 1.4–1.5 times less than those of the other known kHz QPO sources. The estimated magnetic fields for both Z sources (about Eddington accretion rate ) and Atoll sources (∼ 1% ) are approximately ∼109 G and ∼108 G, respectively. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
We show that the interaction of an accretion disk with the magnetosphere of a neutron star can excite resonant shear Alfven waves with Hz-kHz frequencies in a region of enhanced density gradients. This is the the region where accretion material flows along the magnetic field lines in the magnetosphere. We argue that due to the pressure anisotropy produced by the plasma flow, firehose instabilities are likely to occur. Furthermore, for a dipolar field topology, we show that a new instability develops due to both magnetic field curvature and the plasma flow.
