​Earlier this year, an international team of scientists announced the second detection of a gravitational-wave signal from the collision of two neutron stars. The event, called GW190425, is puzzling: the combined mass of the two neutron stars is greater than any other observed binary neutron star system. The combined mass is 3.4 times the mass of our Sun.
 
A neutron star binary this massive has never been seen in our Galaxy, and scientists have been mystified by how it could have formed—until now. A team of astrophysicists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) think they might have the answer.
 
Binary neutron stars emit gravitational waves—ripples in space-time— as they orbit each other, and scientists can detect these waves when the neutron stars merge. The gravitational waves contain information about the neutron stars, including their masses.
 
The gravitational waves from cosmic event GW190425 tell of a neutron star binary more massive than any neutron star binary previously observed, either through radio-wave or gravitational-wave astronomy. A recent study led by OzGrav PhD student Isobel Romero-Shaw from Monash University proposes a formation channel that explains both the high mass of this binary and the fact that similar systems aren’t observed with traditional radio astronomy techniques.

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​In a study recently published in the Monthly Notices of the Royal Astronomical Society, researchers Dr Jade Powell and Dr Bernhard Mueller from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) simulated three core-collapse supernovae using supercomputers from across Australia, including the OzSTAR supercomputer at Swinburne University of Technology. The simulation models—which are 39 times, 20 times and 18 times more massive than our Sun— revealed new insights into exploding massive stars and the next generation of gravitational-wave detectors.
 
Core-collapse supernovae are the explosive deaths of massive stars at the end of their lifetime. They are some of the most luminous objects in the Universe and are the birthplace of black holes and neutron stars. The gravitational waves—ripples in space and time—detected from these supernovae, help scientists better understand the astrophysics of black holes and neutron stars.

​​Future advanced gravitational-wave detectors, engineered to be more sensitive, could possibly detect a supernova—a core-collapse supernova could be the first object to be observed simultaneously in electromagnetic light, neutrinos and gravitational waves.

To detect a core-collapse supernova in gravitational waves, scientists need to predict what the gravitational wave signal will look like. Supercomputers are used to simulate these cosmic explosions to understand their complicated physics. This allows scientists to predict what the detectors will see when a star explodes and its observable properties.

In the study, the simulations of three exploding massive stars follow the operation of the supernova engine over a long duration—this is important for accurate predictions of the neutron star masses and observable explosion energy.
 
OzGrav postdoctoral researcher Jade Powell says: ‘Our models are 39 times, 20 times and 18 times more massive than our Sun. The 39-solar mass model is important because it’s rotating very rapidly, and most previous long duration core-collapse supernova simulations do not include the effects of rotation’.


The two most massive models produce energetic explosions powered by the neutrinos, but the smallest model did not explode. Stars that do not explode emit lower amplitude gravitational waves, but the frequency of their gravitational waves lies in the most sensitive range of gravitational wave detectors.
 
‘For the first time, we showed that rotation changes the relationship between the gravitational-wave frequency and the properties of the newly-forming neutron star,’ explains Powell.
 
The rapidly rotating model showed large gravitational-wave amplitudes that would make the exploding star detectable almost 6.5 million light years away by the next generation of gravitational-wave detectors, like the Einstein Telescope.  

​A team of researchers from the ARC Centre of Excellence for Gravitational Wave discovery (OzGrav) recently published a study revealing something unexpected about black holes: that intermediate-mass black holes with precessing orbits should be easier to detect than standard ones.​

Black holes are regions of space-time from which nothing can escape, not even light. They are the corpses of dead stars that collapsed under their own weight, after running out of fuel. As archaeology helps us to understand how dinosaurs lived, the study of black holes helps us to understand how stars formed, evolved and died.

When two blackholes collide, they release incredible amounts of energy in the form of gravitational-waves (ripples in the fabric of space-time), producing the most powerful space-time storms. By observing these waves, scientists can explore the most fundamental properties of gravity.

Black holes can be classified according to their mass—two different kinds have been identified: black holes with a mass several times bigger than the Sun; and supermassive black holes that lie in the centre of most galaxies (the largest type of black hole), containing a mass billions of times the mass of the Sun.

Intermediate-mass black holes are the elusive missing link; despite the indirect evidence for their existence, scientists have not yet confirmed a conclusive observation of these black holes. Finding them would help to explain the mystery of how stellar mass black holes can evolve into supermassive ones.

The LIGO and Virgo collaboration searched for intermediate-mass black hole collisions during their first and second observing runs, from 2015 to 2018, but were not successful. On the bright side, the lack of a detection allows scientists to confirm how many of these collisions happen in the Universe.

To achieve this in the study researchers including  OzGrav Alumnus Juan Calderon Bustillo, first determined the observable distance of these collisions using supercomputer simulations. The gravitational-wave signals generated from the collision were recorded and injected into the data to assess their recovery rate in the search algorithms.

When doing similar studies, scientists have always assumed that two colliding black holes approach each other with a constant orbital plane, like the orbit of the Earth and the Sun. However, there is another possible situation in which the black holes move up and down, describing what is known as a precessing orbit.

The researchers found that this kind of collision can be observed from a further distance, allowing them to better constrain the number of black hole collisions out there. This also means that these black holes may be easier to detect if they follow precessing orbits rather than standard ones, making them better candidates for a first detection of intermediate-mass black holes. 

​Scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) reveal an alternative explanation of the recently announce black hole merger. The paper was just accepted by Astrophysical Journal Letters.
 
On the 12th of April 2019, the LIGO and Virgo observatories detected gravitational waves—ripples in space and time—from an unusual cosmic event of two black holes merging. Unlike the ten previously reported black hole mergers, in which the two black holes may have had equal or nearly-equal masses, this event, called GW190412, definitely had two very unequal black holes, with the heavier one possibly three or four times more massive than the lighter one.
 
In addition, the discovery paper (released in Australia on 19 April 2020) reported that at least one of the merging black holes had to be spinning: rotating around its axis. However, gravitational waves do not allow accurate measurement of individual spins.  Only a specific spin combination can be measured. Therefore, to infer individual spins, assumptions must be made based on scientific models. The LIGO and Virgo collaborations assumed that the heavier, first-born black hole could be spinning, and reported that it had a moderate spin in the gravitational-wave discovery paper.
 
Within 24 hours of the discovery’s announcement, OzGrav Chief Investigator Ilya Mandel, from Monash University, and collaborator Tassos Fragos, from the University of Geneva, wrote a follow-up paper which has just been accepted by Astrophysical Journal Letters. Motivated by the best current models of the evolution of massive stars in binaries, Mandel and Fragos argued that the more massive, or ‘heavier’, black hole in the event is very slowly spinning; whereas the ‘lighter’ black hole is spinning very fast, in the same direction as the orbital motion.
 


Mandel and Fragos state that if isolated pairs of stars orbiting around each other give birth to merging black holes, they naturally make first-born, heavier black holes that spin very slowly. Before a star forms a black hole, it evolves into a giant with a gaseous envelope.  When it does so, it slows down, like a spinning figure skater extending her arms.  When this envelope is stripped off by extreme tidal forces exerted by the other star in the binary, a slowly rotating central core is left behind, which ultimately collapses into a slowly spinning black hole.
 
The same process should typically apply to the second-born, lighter star, which eventually collapses into the lighter black hole.  However, when the second star loses its gaseous envelope, the binary separation can be sufficiently small enough to allow the naked star core to spin up through ‘tidal locking’.
 
Mandel explains: ‘Tidal locking occurs when tides from an orbiting companion forces an object’s period of rotation—the time it takes it to spin around its axis—to equal the time it takes for a full orbit of the binary system. For example, tidal locking of the Moon to the Earth sets the Moon to rotate the same 28 days equal to its orbital period around the Earth. This explains why we never get to see the dark side of the Moon—except when listening to Pink Floyd.’
 
So, sometimes the second black hole can spin up and rapidly rotate. Mandel and Fragos find this to be the case in the GW190412 event. Such systems should also merge soon after their formation, since tidal locking will only happen in very tight binaries.
 
Although it’s difficult to confirm this interpretation, future detections of black hole mergers will allow for more accurate testing of this model.