Modelling Glitchy Pulsars

You have probably never heard of the pulsar PSR J0537-6910, but even in the strange world of rapidly rotating, dense neutron stars, it is pretty special. PSR J0537-6910 is a millisecond pulsar located in the Large Magellanic Cloud, a galaxy just 163,000 light years away and one of our nearest galactic neighbours. It spins at a rapid 62 revolutions per second, throwing out radiation that sweeps periodically over the Earth like an inter-galactic lighthouse beam. Like all pulsars its spin rate is slowing as it ages, probably because the energy emitted by its rotating magnetic field is having a kind of braking effect. But PSR J0537-6910 is particularly interesting because every now and then its gradual spin-down is interrupted momentarily by a glitch where it spins up in an instant. And this spin-up has been recorded by scientists nearly 50 times.

PSR J0537-6910 is not the only pulsar that has been observed exhibiting this unusual glitchy behaviour, but there are a total of only six pulsars with more than 15 recorded glitches and PSR J0537-6910 is on top of the leaderboard. We do not know exactly what causes the glitches, but they are likely to be a manifestation of what is happening inside the pulsar. It could be that the crust of the star is releasing stress in a star quake. Or perhaps there is a transfer of angular momentum from the fluid interior to the slowing crust. Either way, it appears that stress builds up until it reaches some threshold value and causes the pulsar to glitch. The glitches are a window into the physics occurring in the interiors of these stars.

Last year researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) published a paper in the highly rated journal Nature about the first ever live observations of a pulsar glitching. The pulsar in question was the Vela pulsar, one of the brightest radio pulsars in the sky. The observations were made using the University of Tasmania’s 26-m radio telescope at the Mount Pleasant Radio Observatory in Cambridge. Without making assumptions about the physical processes taking place inside the pulsar, the researchers were able to develop a model of the glitch dynamics. They could then make inferences about what was going, or not going on, inside of the star.

Motivated partly by this study, Julian Carlin, a PhD student from Melbourne University, and Dr. Andrew Melatos, also from Melbourne University, teamed up at OzGrav to build a meta-model – a more abstract model of the physical model – to describe the glitches and make predictions about their occurrences and magnitudes. Their meta-model assumes that the stress evolves in the star fairly randomly until a glitch is triggered. Known as a Brownian model, it differs from other meta-models that don’t allow for the random fluctuations in stress between glitches that might occur due to random processes in the star’s interior. Using their meta-model Mr. Carlin and Dr. Melatos can predict long term statistics relating to pulsar glitches, and from this determine which physical processes are likely to be causing the glitches.

Julian Carlin explains. “We call our modelling in this work a meta-model as we are trying to understand a mathematically simpler system that hopefully encompasses the complicated reality of what’s really happening in the neutron star. Finding predictions from directly modelling the physical process is much harder to do.” One prediction of the Brownian model is that there should be a correlation between big glitches and the time until the next glitch. “If a lot of stress is released, it takes longer on average for the pulsar to build up enough stress for another glitch”, says Carlin.

Having clear and testable predictions like this meant that they could look at the data from a number of glitching pulsars, including PSR J0537-6910 and Vela, to determine whether their meta-model encompassed the physics at work in the pulsars. Interestingly, their model was falsified by some, but not all, of the stars they looked at, indicating that the physical processes underlying glitches may vary from pulsar to pulsar. But to make definitive statements, more glitches need to be observed in the future.

When asked about the importance of this work, Carlin explained that it is not all about glitching pulsars; the meta-model framework they developed can be transferred over to other domains as well. “It can be re-tooled to try to understand any process in which stress builds up and releases in a seemingly random fashion. Some potential extensions for the work include trying to understand solar flares, magnetar bursts, or even things closer to home such as avalanches or bushfires.” More evidence, if any was needed, of how research by Australia’s astronomers can benefit the community in other tangible ways.