Gravitational waves from magnetically induced thermal neutron star mountains

Abstract

Many low-mass X-ray binary (LMXB) systems are observed to contain rapidly spinning neutron stars. The spin frequencies of these systems may be limited by the emission of gravitational waves. This can happen if their mass distribution is sufficiently non-axisymmetric. It has been suggested that such 'mountains' may be created via temperature non-axisymmetries, but estimates of the likely level of temperature asymmetry have been lacking. To remedy this, we examine a simple symmetry breaking mechanism, where an internal magnetic field makes the thermal conductivity tensor direction-dependent. We find that the internal magnetic field strengths required to build the mountains of the necessary size are very large, about four orders of magnitude larger than the inferred external field strengths. We also examine how non-axisymmetric surface temperature profiles, as might be caused by magnetic funnelling of the accretion flow, lead to internal temperature asymmetries, but find that for realistic parameters the induced non-axisymmetries are very small. We conclude that, in the context of this work at least, very large internal magnetic fields are required to generate mountains of the necessary size.

Gravitational-wave Constraints on the Equatorial Ellipticity of Millisecond Pulsars

“…Several such accretion-specific deformation mechanisms are known. One possibility was first noted by Bildsten (1998), who argued that temperature asymmetries in the crust of an accreting star would produce lateral variations in the locations of the transition layers between one nuclear species and the next, a suggestion that has been examined in more detail since (Ushomirsky et al 2000;Singh et al 2020;Osborne & Jones 2020). Another possibility is that the accretion process 'buries' the star's magnetic field, so that a very strong internal field, much larger than the external field of ∼ 10 9 G inferred from a typical recycled pulsar, distorts the star (Vigelius & Melatos 2009).…”

The impact of superconductivity and the Hall effect in models of magnetised neutron stars

“…Understanding the secular evolution of the magnetic field is crucial to connect different evolutionary tracks and NS classes like millisecond pulsars, rotation-powered pulsars, and magnetars. On dynamical timescales, however, it is of interest to understand the physical conditions that allow to obtain stable equilibria in NSs, in order to use such models as backgrounds to model phenomena such as gravitational wave emission due to oscillations or deformations of the crust (Ushomirsky et al 2000;Payne & Melatos 2006;Osborne & Jones 2020;Singh et al 2020).…”

Geometric Approach to Analytic Marginalisation of the Likelihood Ratio for Continuous Gravitational Wave Searches

“…Searches of data from the LIGO and Virgo observatories, most recently from their second [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and third observing runs [21,22], have yet to make a first detection. Theoretical modelling of rapidly-rotating, non-axisymmetric neutron stars-the most likely source of continuous waves-predict a wide range of possible signal strengths [23][24][25][26][27][28][29][30]. Optimally-sensitive data analysis techniques are, therefore, important.…”