Astronomers narrow in on closing the mass gap

We report the observation of a compact binary coalescence involving a 22.2 – 24.3 M_☉ black hole and a compact object with a mass of 2.50 – 2.67 M_☉

On 14 August 2019, the entire Earth stretched and squeezed itself as a passing wave distorted the reality we occupy - and no one on the planet felt any different.

In the US states of Washington and Louisiana, and just outside the city of Pisa in Italy, a particularly loud chirping signal reached three gravitational-wave detectors known as LIGO and Virgo – and recorded a change barely 1/1000th the diameter of a proton.

By coincidence, it would be exactly two years to the day since the first-ever observation of a gravitational-wave signal was received by all three interferometers. That event, in 2017, was classified as a coalescing pair of black holes, but this new event recorded in August 2019 was far more mysterious: a black hole had merged with an unknown astrophysical object.

This new signal, a chirp found in amongst the terabytes of data recorded by the observatories was relatively tiny - but the distinct waveform stood out dramatically, identified against the background noise like a bright neon light in the middle of a vast desert plain. That is when scientists came to the stunning realisation that this event, called GW190814, was unlike any that had been seen before.

It’s not the first time that scientists have detected gravitational-wave signals. In fact, the first was found in September 2015, confirming a 100-year-old theory set out by Einstein and built upon by astrophysicists since.

But now, astronomers from the global LIGO and Virgo collaboration have announced that one of the most explosive and fascinating gravitational-wave events ever witnessed, is also one of the most mysterious. And as a leader in gravitational-wave research, the ARC Centre of Excellence for Gravitational Wave Discovery, OzGrav, an Australian government-funded organisation with ties to several top Australian universities, is playing a leading role in working out just what is going on.

Compact massive objects have eluded scientists for decades. The first observational evidence for them was about 50 years ago when pulsars, a type of neutron star that emits twin beams of radiation from its poles, was discovered by Dame Jocelyn Bell Burnell in 1967. The first strong black hole candidate, called Cygnus X-1, was discovered in 1972 by Bolten, Webster, and Murdin.

Back in 1916, Albert Einstein had predicted that two massive accelerating objects would disrupt space-time in such a way that waves would propagate away in all directions through the fabric of space. This deduction came from a purely mathematical analysis – Einstein had found the ripples in space-time within the equations of his theory of General Relativity.

It would take scientists another 58 years to find a binary system with two neutron stars, one of them a pulsar, that could be used to test Einstein’s theory and determine if gravitational waves were in fact real.

By timing the signal arrival time of pulses, scientists could accurately work out how much the distance between the two massive objects was decaying per orbit as they spiraled towards each other – releasing gravitational waves as a consequence of this acceleration.

The discovery of this pair of neutron stars (known as the PSR B1913+16 system) resulted in a Nobel Prize in Physics for Russell Hulse and Joseph Taylor, but just as importantly it allowed astronomers to verify the predictions of general relativity. Observations over several years showed that the two neutron stars were emitting energy as gravitational waves exactly as predicted by General Relativity.

Similar effects were found in other binary neutron star systems over the years, however, the evidence for gravitational waves was still mathematical and indirect. But things were about to change.

On 14 September 2015, in one of the greatest scientific achievements in history, gravitational waves were directly measured by both LIGO detectors in the US, as the distortions in space-time created by the collision between two black holes over 1.4 billion light-years away passed over the Earth.

Despite the tremendous energy of the event, by the time the waves traversed the vastness of space, the cosmic vibrations were not even enough to stretch the distance from the Earth to the Sun by the size of an atom. But the signal was unmistakably there. Finally, this was a real confirmation of one of the predictions of Einstein’s theory of general relativity, almost 100 years later.

Detecting distortions in space-time might seem like science-fiction, but the idea behind the operation of current generation gravitational-wave detectors is relatively simple. They are based on the Michelson Interferometer, invented in the 19^th century.

Build two long straight arms in an L-shape and fire a laser that gets split by a beamsplitter into each arm. Bounce the light off mirrors at the ends of the arms so that it returns along the same path. Recombine the signals, and analyse the result.

If the waveform is essentially a flat line, nothing happened… But if there is a complex series of peaks and troughs, it is possible that the arms have been stretched or compressed by the passage of a gravitational wave.

Of course, real detectors are much more complicated than that. The engineering requirements alone are astounding. The arms, for example, need to be as long and straight as possible to increase sensitivity, but over the 4-km span of the arms at LIGO the Earth curves away by almost 1-m.

And being so sensitive to gravitational waves also means being sensitive to other spurious noise.

Indeed, this is one of the primary reasons for using multiple facilities that are not located near each other – any Earth-based background noise should be different at each location, and so can be removed from the signals during analysis. That, and scientists use the time delay in the detection of the events between facilities to pinpoint the source in the sky.

When you consider that the causes of the gravitational waves that we are measuring are often hundreds of millions or even billions of light-years away, it is incredible that we can detect anything at all. But colliding black holes and merging neutron stars are some of the most energetic events in the universe, releasing far more energy in a split second than the Sun will emit during its entire life.

While it should be possible to observe the gravitational-waves emitted during core-collapse supernovae in the future, the current generation of gravitational-wave detectors are not quite sensitive enough. Astronomers have, up until now, been able to neatly classify all the gravitational-wave progenitors as either black holes or neutron stars.

And there is a reasonably clear distinction between the two. A neutron star is the collapsed core of a giant star, the result of a supernova explosion that blows away huge amounts of gas while the remainder collapses gravitationally into a sphere the size of a city. The star survives in this state only because repulsive nuclear forces at the sub-atomic level keep the neutrons it is now mostly made of from coming closer together.

However, if the remnant is still massive enough, gravity will force it to collapse further into a black hole. And herein lies an interesting point – there has to be an upper limit to the mass of a neutron star. Once that limit is reached, another teaspoon-full of atoms and there is just too much mass to stand up to the force of gravity.

This limit is known as the Tolman–Oppenheimer–Volkoff limit. Its exact value is difficult to determine theoretically due to our limited knowledge about the behaviour of extremely dense matter, but it is thought to be somewhere between about 1.5 and 2.5 times the mass of the Sun.

While black holes can theoretically exist at any mass (even a human can be made into a black hole if you were able to crush one into a tiny enough space), the gravitational-wave events and data thus far observed by LIGO and Virgo have showcased pairs of neutron stars with masses below 2.5 times the mass of the Sun for neutron star mergers, or black holes with masses of at least 5 times the mass of the Sun for black hole merger events.

Prior to 14 August 2019 there had been no detections of any merger events which indicated that any of the progenitor masses were above that of the heaviest neutron star (2.5 solar masses) and lower than the lightest black hole (5 solar masses).

Within 20-mins of its detection in data from LIGO and Virgo's third observing run, known as O3, GW190814 was announced to the public with the classification of a potential mass gap event - meaning that at least one of its compact objects was estimated to have a mass between 3 and 5 times the mass of the Sun.

Further analysis brought that estimate down to between 2.5 and 3 times the mass of the Sun, still heavier than the heaviest known neutron star and well below the typical mass of a black hole. The source classification was changed to a neutron star-black hole merger, and the event was calculated to have happened about 800 million light-years away.

If GW190814 eventually does turn out to be a merger of a neutron star and a black hole, not only is it the first such event to be observed, but it will also challenge existing theories that fail to explain how neutron stars can get so big.

But there’s more. In all the mergers observed by LIGO and Virgo so far, the two progenitors have had roughly similar masses. The heavier progenitor object in GW190814’s system though was about nine times more massive than its companion, which is considered an anomaly with reference to existing observations.

Merging pairs with such a discrepancy in mass are not often seen in computer simulations, so this discovery implies that they are much more common than had been predicted. At least this mass discrepancy meant that astronomers could accurately calculate the spin of the massive black hole, due to it dominating the spin of the whole system, but even that turned out to be very low.

As far as the study of the mass of compact objects and the processes that lead to their mergers goes, GW190814 is a veritable treasure trove of secrets waiting to be unlocked by astronomers.

And Australia is not only expected to play an important role in figuring out the answers but has also been an integral part of the analysis of GW190814 so far. The supercomputer cluster at the Swinburne University of Technology in Melbourne, called OzStar, which spends more than one-third of its time on gravitational-wave research, has already been at work analysing the signal from GW190814.

“Figuring out the origin of these gravitational waves required using thousands of computers for several months to churn through all the data. Gravitational-wave astronomy is at the bleeding edge of supercomputing, and Australia is a world leader in our field”, says Dr. Rory Smith, a research fellow from the School of Physics and Astronomy at Monash University in Victoria, and an astronomer at OzGrav.

Swinburne and OzGrav Ph.D. student Debatri Chattopadhyay has been using supercomputers to simulate the formation, evolution, and merger of binary systems, and was struck by the uniqueness of this event.

“A couple of things make this event unique: mergers of black holes or neutron stars seem to prefer mass companions that are similar in mass, like birds of a feather. This event has a black hole that is ten times more massive than its partner. The less-massive partner also has a mass range about 2.5-2.8 times the mass of the sun which falls in the mass gap region — the apparent interval between neutron stars and black holes.”

It's also difficult (at this stage) to explain the underlying physics, as Dr. Terry McRae from the Australian National University and OzGrav points out. “This event challenges current models of astrophysics and has implications for cosmology and perhaps even for particle physics. The high mass-ratio of the system also forms part of a parameter space previously unexplored with the theory of general relativity.”

The difference in masses between the two objects was a point also emphasised by OzGrav and the University of Melbourne postdoctoral researcher, Dr. Hannah Middleton. “In past observations, the masses of the two objects colliding have been fairly similar to each other. Heavy things merge with heavy things and light things merge with light things — until now! It looks like GW190814 is the most asymmetric collision so far.”

Because of this, Chattopadhyay hypothesised that the system was more likely to have formed within a dense star cluster than in isolation. “In these dense environments, stars interact with each other more often. They form and break star pairs throughout their lives. This can create mergers between objects with very different masses.”

With all the talk about formation environments and merger scenarios, it is easy to overlook that we are talking about potentially the biggest neutron star ever discovered. The gravity of this event (puns included) was put succinctly by Dr Smith.

“It’s quite difficult to explain why a star so heavy and dense doesn’t just collapse into a black hole, so its very existence hints that we might need new physics in order to understand it. On the other hand, if it turns out to be a black hole, it’s possible we caught a glimpse of a very turbulent, ancient region of the Universe where lots of stars were forming and colliding to produce black holes — even then we would have witnessed a very rare spectacle. It’s an exciting and unexpected astronomical discovery all round!”

What Happens Now?

Just minutes after the detection of GW190814 the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope (operated and managed by Australia's leading science agency, CSIRO) in the Mid West region of Western Australia was tasked to look for an afterglow of the merger.

Leading the team was Professor Tara Murphy from the University of Sydney and OzGrav, with Ph.D. student Dougal Dobie also involved.

“ASKAP is the only radio telescope in the world that can look at the region the gravitational waves come from in a single, highly sensitive, observation. This is the first time anyone has been able to do an untargeted search for the radio afterglow of the merger of two incredibly dense objects.”

Other global telescopes around the world and in space also joined in, slewing to observe the target as it happened and still continue to monitor the region today. So far, no further detections of any nature, have been found.

“Although there was an extensive electromagnetic follow-up by space and ground-based telescopes, the large distance of the event made this challenging,” explained Dr. Eric Howell from the University of Western Australia and OzGrav.

“It’s worth noting that any light signal would have required a neutron star being tidally disrupted before the final merger — the inferred mass ratio of two components coupled with the low spin of the black-hole make this scenario highly unlikely, so any detection of associated light would have been very surprising,” he said.

Unfortunately for science enthusiasts and gravitational-wave astronomers alike, LIGO-Virgo's O3 has had to be suspended earlier than planned due to COVID-19, but there may be another surprise in store in the remaining data.

An event detected on 24 September 2019 also looks like a promising mass gap candidate, but apart from that the science community will likely be waiting until the O4 run, which is scheduled for late 2021 or early 2022, for more evidence of heavy neutron stars and light black holes.

Before then, planned upgrades to the sensitivity of the LIGO interferometer are due, and Japan’s own gravitational-wave detector, KAGRA, should be able to join the entirety of the O4 run for the first time - further enhancing the sensitivity to and localisation of gravitational-wave events.

In the meantime, astronomers from Australia and around the world are left with plenty to think about, as they work to unravel the mysteries of the GW190814 event. Was it a heavy neutron star? Was it a light black hole? Or better yet, was it something more mysterious that can change our view of the Universe, forever.