Journal Article

Dynamic migration of rotating neutron stars due to a phase transition instability

Harald Dimmelmeier, Michal Bejger, Pawel Haensel and J. Leszek Zdunik

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 396, issue 4, pages 2269-2288
Published in print July 2009 | ISSN: 0035-8711
Published online July 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.14891.x
Dynamic migration of rotating neutron stars due to a phase transition instability

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Using numerical simulations based on solving the general relativistic hydrodynamic equations with the CoCoNuT code, we study the dynamics of a phase transition in the dense core of isolated rotating neutron stars, triggered by the back bending instability reached via angular momentum loss. In particular, we investigate the dynamics of a migration from an unstable configuration into a stable one, which leads to a mini-collapse of the neutron star and excites sizeable pulsations in its bulk until it acquires a new stable equilibrium state. We consider two equations of state which exhibit softening at high densities, a simple analytic one with a mixed hadron-quark phase (where the hadron pressure is approximated by a polytrope) in an intermediate pressure interval and pure quark matter at very high densities, and a microphysical one that has a first-order phase transition at constant pressure with a jump in density, originating from kaon condensation. Although the marginally stable initial models are rigidly rotating, we observe that during the collapse (albeit little) differential rotation is created. We analyze the emission of gravitational radiation in such an event, which in some models is amplified by mode resonance effects, and assess its prospective detectability by current and future interferometric detectors. We expect that the most favourable conditions for dynamic migration exist in very young magnetars. The rate of such events in our Galaxy is of the order of one per century and rises to about one per year if the Virgo cluster of galaxies is considered. We find that the damping of the post-migration pulsations and, accordingly, of the gravitational wave signal amplitude strongly depends on the character of the equation of state softening (either via a density jump or continuous through a mixed state). The damping of pulsations in the models with the microphysical equation of state is caused by dissipation associated with matter flowing through the density jump at the edge of the dense core. If at work, this mechanism dominates over all other types of dissipation, like bulk viscosity in the exotic-phase core, gravitational radiation damping or numerical viscosity.

Keywords: hydrodynamics; relativity; methods: numerical; stars: neutron; stars: rotation

Journal Article.  15522 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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