Tangled magnetic field model of QPOs

Abstract

The highly tangled magnetic field of a magnetar supports shear waves similar to Alfvén waves in an ordered magnetic field. Here we explore if torsional modes excited in the stellar interior and magnetosphere can explain the quasi-periodic oscillations (QPOs) observed in the tail of the giant flare of SGR 1900+14. We solve the initial value problem for a tangled magnetic field that couples interior shear waves to relativistic Alfvén shear waves in the magnetosphere. Assuming stellar oscillations arise from the sudden release of magnetic energy, we obtain constraints on the energetics and geometry of the process. If the flare energy is deposited initially inside the star, the wave energy propagates relatively slowly to the magnetosphere which is at odds with the observed rise time of the radiative event of $\lesssim 10$ ms. Nor can the flare energy be deposited entirely outside the star, as most of the energy reflects off the stellar surface, giving surface oscillations of insufficient magnitude to produce detectable modulations of magnetospheric currents. Energy deposition in a volume that straddles the stellar surface gives agreement with the observed rise time and excites a range of modes with substantial amplitude at observed QPO frequencies. In general, localized energy deposition excites a broad range of modes that encompasses the observed QPOs, though many more modes are excited than the number of observed QPOs. If the flare energy is deposited axisymmetrically, as is possible for a certain class of MHD instabilities, the number of modes that is excited is considerably reduced.

19 pages, 9 figures, published in MNRAS

References in corpus (17)

• Corona of Magnetars
• Sound velocity bound and neutron stars
• QPOs during magnetar flares are not driven by mechanical normal modes of the crust
• On non-axisymmetric magnetic equilibria in stars
• On the theory of magnetar QPOs
• Elastic or magnetic? A toy model for global magnetar oscillations with implications for QPOs during flares
• Alfvén QPOs in Magnetars
• Magnetars oscillations in the presence of a crust
• Entrainment parameters in cold superfluid neutron star core
• Constraints on the Magnetic Field Geometry of Magnetars
• The first XMM-Newton observations of the Soft Gamma-ray Repeater SGR 1900+14
• Quasi-Periodic Oscillations in Short Recurring Bursts of the Soft-Gamma Repeater J1550-5418
• Quasi-Periodic Oscillations in Short Recurring Bursts of the magnetars SGR 1806-20 and SGR 1900+14 Observed With RXTE
• Composition temperature-dependent g-modes in superfluid neutron stars
• Powering Central Compact Objects with a Tangled Crustal Magnetic Field
• Quiet but still bright: XMM-Newton observations of the soft gamma-ray repeater SGR 0526-66
• Modulating the magnetosphere of magnetars by internal magneto-elastic oscillations

Cited by in corpus (5)

• Evidence for an abundant old population of Galactic ultra long period magnetars and implications for fast radio bursts
• Non-Axisymmetric Precession of Magnetars and Fast Radio Bursts
• Search for Long-duration Gravitational-wave Signals Associated with Magnetar Giant Flares
• Ultraslow PSR J0901-4046 with an ultrahigh magnetic field of $3.2\times 10^{16}$ G
• Estimation of neutron star mass and radius of FRB 20240114A by identification of crustal oscillations