Journal Article

On the quasi–periodic oscillations in magnetars

A. Colaiuda, H. Beyer and K. D. Kokkotas

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 396, issue 3, pages 1441-1448
Published in print July 2009 | ISSN: 0035-8711
Published online June 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.14878.x
On the quasi–periodic oscillations in magnetars

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We study torsional Alfvén oscillations of magnetars, that is neutron stars with a strong magnetic field. We consider the poloidal and toroidal components of the magnetic field and a wide range of equilibrium stellar models. We use a new coordinate system (X, Y), where and and a1 is the radial component of the magnetic field. In this coordinate system, the one+two–dimensional evolution equation describing the quasi–periodic oscillations (QPOs), see Sotani et al., is reduced to a one+one–dimensional equation where the perturbations propagate only along the y–axis. We solve the one+one–dimensional equation for different boundary conditions and the open magnetic field lines, that is magnetic field lines that reach the surface and there match up with the exterior dipole magnetic field as well as closed magnetic lines, i.e. magnetic lines that never reach the stellar surface. For the open field lines, we find two families of QPO frequencies: a family of ‘lower’ QPO frequencies which is located near the x–axis and a family of ‘upper’ frequencies located near the y–axis. According to Levin, the fundamental frequencies of these two families can be interpreted as the turning point of the continuous spectrum. We find that the upper frequencies are multiples of the lower ones by a constant equalling 2n+ 1. For the closed lines, the corresponding factor is n+ 1. By using these relations, we can explain both the lower and the higher observed frequencies in SGR 1806−20 and SGR 1900+14.

Keywords: stars: magnetic field; stars: neutron; stars: oscillations

Journal Article.  5354 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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