A stellar object that is lighter than the lightest known black hole, but heavier than the heaviest known neutron star has been discovered for the first time.
The discovery was made by the LIGO gravitational wave observatory and could reshape our understanding of the biggest stars in the universe, researchers claim.
The object sits in what is known as the mass gap – a decades old theory that there should be an object between the mass of a black hole and that of a neutron star.
This object, dubbed a black neutron star, was detected after gravitational waves generated when it collided with a massive black hole reached the Earth.
The LIGO team describe this collision as a cosmic ‘face-off’ between two massive objects 800 million light-years away from Earth.
This black neutron star was in a binary pairing with a large black hole before the two collided
They said these objects would have circled each other before finally colliding and when the big event happened the black hole and black neutron star would have merged to form a single, larger black hole.
This catastrophic event generated a ‘huge splash of gravitational waves that were emitted across the Universe’ that were detected here on Earth.
The scientists are confident that one of the objects was itself a huge black hole roughly 23 times the mass of our sun – but the details of the second were more unusual and harder to confirm.
The event was originally detected in August 2019, but it has taken this long for the researchers to exclude all other options before settling on this new object type.
Professor Alberto Vecchio, director of the Institute for Gravitational Wave Astronomy at the University of Birmingham, said it was a surprising discovery.
In August 2019, the LIGO-Virgo gravitational-wave network witnessed the merger of a black hole with 23 times the mass of our sun and a binary companion 2.6 times the mass of the sun
‘We thought the Universe would be kind of lazy in producing binaries of objects with such different masses, if it did so at all. And guess what, we were wrong!’ he said.
‘We now know there are cosmic factories hiding somewhere that are rather efficient at generating these systems,’ he said.
‘The journey to figure out what they are and how they work is going to keep us busy for quite some time, but more and better data from LIGO and Virgo are just about a year away, and we are bound to have new surprises.’
The new observation is important because it challenges astrophysicists’ understanding both of how stars die and how they pair up into binary systems.
A binary system is a system of two astronomical bodies that are close enough for their gravitational attraction to make them orbit each other around a central point.
The size of the mysterious object in this system lies in what scientists call the ‘mass gap’ and is thought to be something that is heavier than the heaviest known neutron star yet lighter than the lightest known black hole.
Up until now, scientists have not been able to find direct evidence of anything that lies within this mass gap.
The announcement is the latest in a line of spectacular discoveries whereby gravitational waves – tiny ripples that spread through space and time when two massive objects collide – have been detected on Earth.
They are used to paint a picture of some of the biggest, most violent and unusual cosmic events happening across the Universe.
‘When I first saw the alert come through, my jaw hit the floor,’ said Charlie Hoy from Cardiff University, who worked with LIGO when the discovery was made.
‘It was not until I saw the significance of this event that it hit me how important this event could be for astrophysicists around the world,’ he said.
‘This was the first possible detection of a highly significant neutron star-black hole candidate – something that we had previously never seen before.’
Cardiff University has been involved in the US-backed LIGO since its inception and the university’s Gravity Exploration Institute has developed algorithms and software that have become standard tools for detecting gravitational wave signals.
The latest detection of gravitational waves, using sophisticated detectors in the US and Italy, actually took place on August 14, 2019.
This graphic shows the masses of black holes detected through electromagnetic observations (purple), black holes measured by gravitational-wave observations (blue), neutron stars measured with electromagnetic observations (yellow), and neutron stars detected through gravitational waves (orange)
Since then, Hoy and colleagues has been leading the ‘parameter estimation’ for this particular event on behalf of LIGO.
This involved untangling the gravitational wave signals and matching them with millions of possible combinations to determine the properties of the objects that produced the gravitational waves in the first place.
This includes parameters such as their mass, the speed and direction at which they were spinning and their distance from Earth.
‘Being a junior member of the collaboration responsible for a significant chunk of the analysis and the writing of the discovery paper has been a huge learning experience, and one that I am hugely grateful for being part of,’ said Hoy.
The gravitational waves emitted from this particular event, which has been dubbed GW190814, led to the creation of a black hole 25 times the mass of the Sun.
The mysterious object is believed to have had a mass 2.6 times that of the sun, placing it firmly in the so-called ‘mass gap’.
The collision of the two objects generated gravitational waves that spread across the universe and hit the Earth
The two objects – one of which was a black hole 23 times the mass of the Sun – merged to form a larger black hole 25 times the mass of the Sun
‘Whether any objects exist in the mass gap has been an ongoing mystery in astrophysics for decades,’ Hoy continued.
‘What we still don’t know is whether this object is the heaviest known neutron star or the lightest known black hole, but we do know that either way it breaks a record.
‘What is really exciting is that this is just the start. As the detectors get more and more sensitive, we will observe even more of these signals, and we will be able to pinpoint the populations of neutron stars and black holes in the universe.’
Dr Vivien Raymond, a member of the LIGO team based at Cardiff University’s School of Physics and Astronomy, said this new detection pushes boundaries of what we know about neutron stars and black holes.
‘This new event in particular involved joint efforts by many different international experts in the collaboration, and we are trying to get ready for the next surprise nature will reveal,’ said Raymond.
Professor Sheila Rowan, Director of the University of Glasgow’s Institute for Gravitational Research (IGR), said the discovery was nearly missed altogether.
‘It’s possible that that we might have missed it altogether if we hadn’t taken that time to reflect on and learn from our early successes,’ said Rowan.
More cosmic observations and research will need to be undertaken, to establish whether this new object is indeed something that has never been observed before or whether it may instead be the lightest black hole ever detected.
It could also be the heaviest neutron star ever detected, researchers say.
The new findings from the LIGO Scientific Collaboration and the European Virgo Collaboration have been published in The Astrophysical Journal.
WHAT ARE GRAVITATIONAL WAVES?
Scientists view the the universe as being made up of a ‘fabric of space-time’.
This corresponds to Einstein’s General Theory of Relativity, published in 1916.
Objects in the universe bend this fabric, and more massive objects bend it more.
Gravitational waves are considered ripples in this fabric.
Gravitational waves are considered ripples in the fabric of spacetime. They can be produced, for instance, when black holes orbit each other or by the merging of galaxies
They can be produced, for instance, when black holes orbit each other or by the merging of galaxies.
Gravitational waves are also thought to have been produced during the Big Bang.
Scientists first detected the shudders in space-time in 2016 and the discovery was hailed the ‘biggest scientific breakthrough of the century’.
Experts say gravitational waves open a ‘new door’ for observing the universe and gaining knowledge about enigmatic objects like black holes and neutron stars.