Journal Article

Quadrupole moment of a magnetically confined mountain on an accreting neutron star: effect of the equation of state

M. Priymak, A. Melatos and D. J. B. Payne

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 417, issue 4, pages 2696-2713
Published in print November 2011 | ISSN: 0035-8711
Published online November 2011 | e-ISSN: 1365-2966 | DOI:
Quadrupole moment of a magnetically confined mountain on an accreting neutron star: effect of the equation of state

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Magnetically confined mountains on accreting neutron stars are promising sources of continuous-wave gravitational radiation and are currently the targets of directed searches with long-baseline detectors like the Laser Interferometer Gravitational Wave Observatory (LIGO). In this paper, previous ideal-magnetohydrodynamic models of isothermal mountains are generalized to a range of physically motivated, adiabatic equations of state. It is found that the mass ellipticity ε drops substantially, from ε≈ 3 × 10−4 (isothermal) to ε≈ 9 × 10−7 (non-relativistic degenerate neutrons), 6 × 10−8 (relativistic degenerate electrons) and 1 × 10−8 (non-relativistic degenerate electrons) (assuming a magnetic field of 1012.5 G at birth). The characteristic mass Mc at which the magnetic dipole moment halves from its initial value is also modified, from Mc/M≈ 5 × 10−4 (isothermal) to Mc/M≈ 2 × 10−6, 1 × 10−7, and 3 × 10−8 for the above three equations of state, respectively. Similar results are obtained for a realistic, piecewise-polytropic nuclear equation of state. The adiabatic models are consistent with current LIGO upper limits, unlike the isothermal models. Updated estimates of gravitational-wave detectability are made. Monte Carlo simulations of the spin distribution of accreting millisecond pulsars including gravitational-wave stalling agree better with observations for certain adiabatic equations of state, implying that X-ray spin measurements can probe the equation of state when coupled with magnetic mountain models.

Keywords: accretion, accretion discs; stars: magnetic field; stars: neutron; pulsars: general

Journal Article.  13342 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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