Gravitational waves in space reveal the secret lives of dead stars

New research, led by the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav) has highlighted through simulations how a space-based laser interferometer will be able to detect the weaker signals of in-spiral binary neutron stars when the two massive objects are still some distance apart.

A double neutron star system comprises of two neutron stars orbiting each other, produces periodic disturbances in the surrounding space-time, much like ripples spreading on a pond surface. These ‘ripples’ are called gravitational-waves.

The study, led by Ph.D. student Mike Lau, predicts that gravitational waves from binary neutron star systems, along with binary white dwarf systems, will be detected by the future space-based observatory, known as the Laser Interferometer Space Antenna (LISA) mission.

The paper is a collaboration of OzGrav working with Monash Univerity and Swinburne University of Technology, along with international partners at the University of Birmingham, University of Copenhagen, and Universitá degli studi Milano Bicocca. The results were published in the Monthly Notices of the Royal Astronomical Society,

Lau, the first author of the paper, compares his team to ‘astro-paleontologists’: “[It’s] Like learning about a dinosaur from its fossil, we piece together the life of a binary star from their double neutron star fossils,” he said.

Using computer simulations to model a population of double neutron stars, the team predicts that in four years of operation, LISA will have measured the gravitational waves emitted by 35 double neutron star systems, as they orbit each other. From this number, 94% are predicted to be from within the Milky Way Galaxy, 5% from the Large Magellanic Cloud (LMC) and 1% from the Small Magellanic Cloud (SMC) over a four-year period.

Both the LMC and SMC are neighbouring galaxies which can be seen with the naked eye from southern locations, such as Australia. They reside approximately 160,000 and 200,000 light-years away from our Galaxy.

In detail, however, the research highlights that LISA will be able to detect weaker gravitational-wave distortions in space-time, relative to Earth-based interferometers like the Laser Interferometry Gravitational-wave Observatory (LIGO), opening the door to potential new discoveries in astrophysics.

To date, scientists still haven’t found a way to measure the gravitational waves given off when two neutron stars or black holes are still far apart in their orbit. These weaker waves hold valuable information about the lives of stars and could reveal the existence of entirely new object populations in our Galaxy.

Neutron stars are on the extreme end of the scale of exotic objects that reside in our Universe. Formed during the violent death of only the most massive stars, they defy all logic that forms a part of everyday objects like our Sun, Moons, comets, planets and even regular stars.

When massive stars reach the end of the main sequence phase of their lives (during which hydrogen is fusing in the core to produce energy), they transform into compact remnant objects – and the higher the mass of the progenitor star, the more extreme the remaining object will be.

Some stars, like the Sun, will puff off outer layers leaving the hot, planet-sized cinder known as a white dwarf star. Yet, bigger stars produce a more compact object – one the size of a city, with many times the density. This is called a neutron star. A teaspoon of this material would weigh as much as a cube of Earth, with sides measuring 800m³.

The culprit to a neutron star’s massive density can be allocated to the moment the progenitor star detonates in a supernova explosion. During this explosion, whilst enormous amounts of the star’s material is thrown into space, a small portion of it also collapses inwards towards the core, due to the pull of gravity.

This collapse is so strong, that it forces electrons to be squeezed and combined together with protons, forming the more massive neutrons particles. Colossal amounts of material are squeezed into a small space, and as such mass, density, gravity, angular momentum, and magnetic field flux surge into the extremities.

Some neutron stars spin rapidly and emit beams of radio waves from their poles. Where this is the case, these are known as pulsars.

For some stars, who are originally even bigger still (roughly 25 times the mass of our Sun and upwards) – not even neutron stars can stop the inward collapse of gravity. In these scenarios – black holes form – regions of space-time where not even light itself can escape.

It’s well known that most of the stars in our Galaxy (and Universe) come in binary pairs. So it is not too uncommon to see stars aging together as they travel through their main sequence phase.

For pairs of massive stars, this becomes interesting. During the supernova explosion of the first star, what happens to the system? Does it blow itself apart? Does it remain as is – now with a compact object like a neutron star, the other the remaining main sequence object? What happens when the second star also then goes supernova?

Interesting questions like these are being researched by astrophysicists around the world through many different forms of wavelengths and signals – some examples:

• By using radio telescopes to study pulsars in binary systems, radio astronomers are able to derive that the massive neutron star (i.e. the pulsar) at one point started drawing matter from its binary partner like a vampire, causing the pulsar to eventually speed up
• By looking at high-energy x-rays from binary star systems, astronomers have been able to indirectly observe main sequence stars being torn apart by elusive and invisible black holes – evidenced by a bright x-ray signature from the accretion disc of the infalling matter being ripped away from the remaining star
• By studying gravitational waves that pass by the Earth, astrophysicists are able to determine the pre and post-merger masses and spin rates of black holes in binary systems using LIGO’s advanced and precise instruments

Binary systems that have massive objects, like neutron stars and black holes are interesting to scientists due to their in-spiral motion – the direct result of gravitational waves extracting energy from the system and radiating away into space.

In some scenarios of binary neutron star systems, the supernova explosion will ‘kick’ one of the stars into a more elongated, oval-shaped orbit (known as eccentricity). Over time, the leaking of gravitational waves will recalibrate itself to circle, so finding these systems when they are most eccentric will provide astrophysicists with a wealth of information, prior to the merger.

So far, the LIGO detectors have confirmed two cases of binary neutron star systems where the two neutron stars have merged after closing in on each other.

The first detected on 17 August 2017 (called GW170817), not only produced a gravitational-wave signature captured by the interferometers but also allowed global scientists to be able to observe a colossally violent event, known as a Kilonova – which produced an electromagnetic (EM) signal.

The EM counterpart signal confirmed that heavy elements like Strontium are produced in such furious events, and provided lots of insights into the outcomes of the gravitational-wave merger signal – the first of its kind (prior detections involved all binary black hole pairs).

The second binary neutron star merger event detected by ground-based interferometers in April of 2019 (called GW190425) has caused astrophysicists to scratch their heads and reconsider existing theories and models. In this latter event, not only was no EM signal counterpart ever detected (unlike the first), the resultant remnant object was too heavy to be a neutron star – weighing in at 3.4 times the mass of our Sun, which is above the threshold of when a neutron star gets too big for its own gravity – and should become a black hole.

Unfortunately, with ground-based interferometers, the signal for such mergers is only ‘loud’ enough to be heard near the final moments of the pair's merger.

To detect a longer-based signal, that is – the weaker gravitational-wave signal – we need to put an interferometer into space.

Space-based observatories have an advantage over their terrestrial counterparts, especially in the higher energy EM spectrum – they don’t have to deal with atmospheric block – where higher wavelength signals from space don’t come through to the surface of Earth (this is a good thing, as life would be very different if we were constantly bombarded with gamma and x-ray radiation).

Ground-based interferometers are also limited in sensitivity of detecting weaker, low-frequency gravitational-wave signals by the seismic noise, localised interference and the size of their arms (LIGO’s detectors are 4 km long and L-shaped perpendicular). However, a space-based interferometer system would allow these low-frequency signals to be detected, complimenting (rather than competing) the gravitational-wave spectrum with LIGO and other ground-based observatories.

That’s where LISA steps in.

Originally designed in the early 1990s, the LISA mission got started with both ESA and NASA, but due to funding limitations, in 2011 – NASA decided to no longer pursue the mission, leaving it in the hands of the Europeans.

LISA is made up of three satellites that form a giant equilateral triangle orbiting along an Earth-like orbit around the Sun. Between each of the satellites, equipment aboard would precisely beam lasers between each three spacecraft, separated by a distance of 2.5 million km each. Mirrors and sensors (known as ‘test-masses’) aboard each spacecraft would continuously monitor the exact length each laser travels from its spacecraft – recording any shift in these exact values as a potential distortion of space-time – a passing gravitational wave.

The dramatic increase in the ‘length’ of the arm from 4km (terrestrial) to 2.5 million-kilometres (space-based) will provide the sensitivity needed to detect the subtle, lower-frequency gravitational waves from binary neutron star systems, just as astrophysicists like Dr. Lau have predicted.

The LISA mission is not expected to launch prior to 2034 with scientists currently working on advancing sensitivity and resolving complex engineering challenges. NASA has also signaled it may re-enter the program, as a junior partner – following the success of ground-based interferometers.

New prototypes for optical lasers to be used in space are being researched, whilst specially-coated mirror surface technology is being tested to withstand the extremities of space and still perform to a degree of accuracy that has not yet been achieved by humans.

For astrophysicists like Dr. Lau, understanding of binary stars—stars that are born as a pair—is plagued with many uncertainties. Scientists hope that by the 2030s, LISA’s detection of double neutron stars will shed some light on their secret lives.