Special Astrophysics BYOL Seminar
(Bring Your Own Lunch)

Gravitational radiation from pulsar glitches:
nuclear physics with LIGO

Andrew Melatos

University of Melbourne

Pulsars are fantastically stable clocks, with spin periods measured to 15 significant figures in some objects. They spin down steadily over millions of years under the action of electromagnetic torques. However, roughly 5% of known pulsars also experience “glitches”, which are tiny, randomly timed, discontinuous spin-up events. The physical origin of glitches remains a mystery after 40 years. Recent radio pulsar timing data, drawn primarily from the Parkes Multibeam Survey, has quadrupled the glitch database and effected a sea change in our ideas about the glitch phenomenon. In this talk, I present the latest data and discuss their implications for the long-standing superfluid vortex paradigm as well as for current and future experiments with the Laser Interferometer Gravitational Wave Observatory (LIGO). It is now clear, from glitch statistics in individual pulsars, that the size and waiting-time distributions of the spin-up events are power-law and Poissonian respectively. I present two collective mechanisms which can account for the observed statistics: vortex avalanches as a self-organised critical process, and sympathetic vortex unpinning as a self-regulated “coherent noise” process. The data raise fundamental challenges for both these mechanisms, including the need for macroscopic inhomogeneity, the gross mismatch between microscopic and observed unpinning rates, and the absence of the size-waiting-time correlation (“reservoir effect”) expected from vortex unpinning. I also present calculations of the periodic gravitational wave signal emitted by nonaxisymmetric Ekman pumping during the relaxation stage (days to weeks) following a glitch. It is shown that the signal, once detected, can be inverted to infer the compressbility and viscosity of bulk nuclear matter. The results of such a detection will be comparable with experimental data from terrestrial relativistic heavy-ion colliders. A brief discussion is given of how the gravitational wave signal and glitch physics are affected by the presence of superfluid turbulence inside a neutron star, both large-scale Kolmogorov-type circulation and small-scale reconnecting vortex tangles.