Neutron star secrets revealed by ‘glitch’

A ‘glitch’ in the behaviour of the Vela pulsar has allowed researchers to glimpse beneath the surface of a neutron star — the densest objects in the known universe — for the first time.

A ‘glitch’ in the behaviour of the Vela pulsar has allowed researchers to glimpse beneath the surface of a neutron star for the first time (NASA/Chandra)

A study published in the journal, Nature Astronomy details the findings of a team from Monash University, the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), McGill University in Canada, and the University of Tasmania, who studied the Vela Pulsar, a neutron star in the southern sky — 1000 light-years from earth.

Neutron stars are the densest objects in the known universe, they are protected from collapsing into black holes by neutron degeneracy pressure, balancing out the inward gravitational pull. They spin rapidly and periodically.

The team took advantage of ‘glitch’ in the rotation of neutron stars — found in only 5% of the objects — which sees them periodically increase the speed of their rotation. This bump in speed — referred to as a ‘spin up’ — is caused by portions of the star’s interior moving outwards.

as such, it gives astronomers a brief glimpse of what lies inside these objects.

Vela is a favourite amongst ‘glitch hunters’ such as the paper’s first author — Dr Greg Ashton, from the Monash School of Physics and Astronomy, and a member of OzGrav — because it is known to ‘glitch’ once every three years.

Despite the regularity, the glitches are still unpredictable. The glitch that the team examined was recorded at the Mount Pleasant observatory in 2016. It was extraordinary brief, lasting only 5 seconds.

By reanalysing data from observations of this event collected by co-author Dr Jim Palfreyman from the University of Tasmania, Dr Ashton and his team found that during the glitch the star actually started spinning even faster still, before relaxing down to a final state.

Dr Lasky — an ARC Future Fellow also from the Monash School of Physics and Astronomy, and a member of OzGrav — remarks that this observation is particularly important because it marks the first time that the scientists glimpsed the interior of the star.

The peak under the surface of Vela revealed that the inside the star actually has three different components.

A glitch in the behaviour of neutron star Vela allowed researchers to glimpse beneath the surface. (Carl Knox)

Dr Lasky explains: “One of these components, a soup of superfluid neutrons in the inner layer of the crust, moves outwards first and hits the rigid outer crust of the star causing it to spin up.

“But then, a second soup of superfluid that moves in the core catches up to the first causing the spin of the star to slow back down.”

Lasky points out that this overshoot has been previously predicted by researchers — including study co-author Dr Vanessa Graber from McGill University, who visited the Monash team as an OzGrav international visitor earlier this year — but it has never been before been identified from observation.

According to Dr Ashton, another observation made by the team, thus far, defies explanation: “Immediately before the glitch, we noticed that the star seems to slow down its rotation rate before spinning back up.

“We actually have no idea why this is, and it’s the first time it’s ever been seen. It could be related to the cause of the glitch, but we’re honestly not sure.”

The team expect that this new paper will inspire some new theories on neutron stars and their glitches.