Special Astrophysics Seminar

Gravitational radiation from accreting neutron stars

Andrew Melatos

University of Melbourne

The spin frequency distribution of neutron stars in low-mass X-ray binaries (LMXBs) exhibits a sharp cut-off well below the centrifugal breakup limit. If the cut-off is interpreted in terms of gravitational-wave stalling, LMXBs should emit a relatively strong, periodic gravitational wave signal detectable by the Laser Interferometer Gravitational Wave Observatory (LIGO). LMXBs enjoy several advantages over binary inspirals: they are persistent, their spin frequency and sky position are often known in advance from X-ray observations, and they emit an approximately “pure tone”. One way to create the necessary nonaxisymmetry is to build up a magnetically confined mountain by accretion onto the poles of the star. In this talk, I present the latest results from theoretical modelling of magnetic mountains. These include: equilibrium states in 2D and 3D and their hydromagnetic stability, the role of resistive relaxation and sinking, global oscillations of the mountain (and a new analytic method for computing the continuous part of the oscillation spectrum), the role of precession, and an updated detectability estimate for LIGO. I report on the status of a radiometer search for magnetic mountains with LIGO, and a search for precession in LMXBs in X-ray timing data from the RXTE satellite. Current LIGO upper limits already place interesting limits on the electrical resistivity of neutron star matter. The magnetic mountain idea can be applied to understand several puzzling astrophysical observations, e.g. the spin down of the accreting millisecond pulsar J1808-3658, the very sinusoidal shape of oscillations in thermonuclear X-ray bursts, and the polarisation swing in millisecond pulsars. I point out several gaps in magnetic mountain modelling which need to be filled in the future.