Putting a New Spin on Neutron Star Analysis

Neutron stars may be some of the most exotic and enigmatic objects in the universe, but over the last few years we have been slowly unravelling their mysteries by studying the gravitational waves that they emit when they merge. Those calamitous events allow astronomers to infer certain properties of the doomed stars, but it can be difficult to distinguish between them when their masses are similar.

Indeed, neutron star mergers that produce the gravitational waves we detect here on Earth are expected to have been precipitated by objects of comparable masses. That’s why a team of researchers, led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), are suggesting a different characterisation based on how fast the objects were spinning.

Neutron stars are the remnants of stars at least around eight times more massive than the Sun. When these behemoths run out of fuel, they begin to collapse, sending temperatures soaring.

At some point the collapse is halted and the outer envelope of the star is flung back outwards by waves of tiny particles called neutrinos. This ends in a supernova, a massive and spectacular explosion where material is blown off the outer envelope of the star leaving only a rapidly spinning compact remnant – a neutron star.

Binary systems are thought to originate in a couple of different ways. One is where the neutron stars in dense star clusters interact with other stars before eventually pairing up and merging. Modelling suggests that this is a relatively uncommon occurrence however.

More commonly they begin with two ordinary stars already in orbit about each other. In this case, the more massive star goes supernova first, but then continues to accumulate matter from its partner and begins spinning faster as a result. The researchers refer to this star as a recycled neutron star.

For the partner star though there is no additional material to accumulate, and so it spins down relatively quickly after its formation. It is known as a slow neutron star, and is effectively non-spinning during the final merger event.

Classifying the stars in this way has its advantages. OzGrav postdoctoral researcher and lead author of the study Xing-Jiang Zhu explains.

“The motivation for proposing the recycled-slow labelling scheme of a binary neutron star system is two-fold. First, it’s a generic feature expected for neutron star mergers. Second, it might be inadequate to label two neutron stars as primary and secondary because they’re most likely to be of similar masses and it’s hard to tell which one is heavier.”

On 17 August 2017 a gravitational wave signal known as GW170817 was observed by detectors at both LIGO and Virgo originating from an elliptical galaxy called NGC 4993. The event was significant for being the first direct detection of a pair of neutron stars as they merged, but it also represented a breakthrough in another respect. Aside from the gravitational waves emitted, astronomers were able to see the intense energy the event produced in gamma rays, microwaves, and even in visible light.

Just seconds after the gravitational waves passed over our detectors, a gamma-ray burst was detected by an orbiting space telescope, and within hours teams from around the world had turned their instruments on the neutron star pairs’ small patch of sky to observe the aftermath. The merger became the most studied event in the history of astronomy, with an average of three scientific papers published every day since.

When the researchers analysed GW170817 using the recycled-slow scheme that they had developed, they found that not only did the two neutron stars have very similar masses but also that the recycled neutron star in the pair had been spinning quite slowly. The lack of spin suggests that the binary probably took billions of years to merge, which is consistent with the galaxy NGC 4993 itself showing no signs of star formation activities over that same period.

They also looked at a more recent binary neutron star merger from 2019 known as GW190425, notable for having the most massive pair of merging neutron stars yet discovered. The recycled neutron star in that event was rapidly spinning, suggesting a relatively quick merger by two stars in a tight orbit.

Most gravitational wave detections to date have been from black hole mergers, but scientists expect many more discoveries of binary neutron stars to be made soon.

Both the LIGO and Virgo detectors are currently undergoing scheduled maintenance and upgrades after completing their joint third observing run earlier this year, and the fourth run is scheduled to begin in the second half of 2021. The new Kamioka Gravitational Wave Detector (KAGRA) is also expected to be operationally ready by then, and will help to provide greater sensitivity to the detection of binary neutron star mergers over a wider range of distances.

“We are in a golden era of studying binary neutron stars with highly-sensitive gravitational-wave detectors that will deliver dozens of discoveries in the next few years,” says Zhu. And that will be vital in helping astronomers to gain a better understanding of these puzzling but fascinating stars.