17th August 2017: a date marked down in the history books—the day the LIGO/Virgo collaboration made the first detection of gravitational waves from the death spiral of two neutron stars. Just 1.7 seconds later, astronomers observed a short burst of high-energy gamma rays known as a gamma-ray burst (GRB). Global efforts by thousands of astronomers later identified the host galaxy and a supernova-like thermal transient called a kilonova. This event gave astronomers insight into several fundamental and important questions, including an unprecedented understanding of where gold and other heavy elements are produced in the Universe, as well as our best measurement of the speed of gravity. Among other things, it confirmed that neutron star mergers originate from short-duration GRBs. Despite the numerous observations, an important question remains unanswered. What was the outcome of this merger?
 
Typically, one expects the merger of two neutron stars to immediately produce a black hole—an object so dense, that light itself cannot escape; however, observations of other GRBs show evidence for the immediate formation of a massive, rapidly-spinning neutron star. Such merger remnants, if they exist, have important implications for the physical composition of neutron stars.
 
Neutron stars are the only place in the Universe where we can study the behaviour of matter at temperatures up to 100 billion times hotter than on Earth and densities greater than an atomic nucleus—these conditions could never be reproduced on Earth. Nikhil Sarin, Paul Lasky, and Gregory Ashton—three researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University—recently published a study analysing all short-duration GRBs observed by NASA’s Neil Gehrels Swift Satellite. Out of 72 GRBs analysed, 18 show evidence for the immediate formation of a massive neutron star which later collapses into a black hole. Combining information from all 18 observations, the team were able to accurately describe the physical composition of these neutron stars.
 
The results indicate that these neutron stars are consistent with having a freely-moving ‘quark’ composition and a composition like regular matter, i.e. composed of atomic nuclei—the building blocks of the Universe. Quarks are elementary particles that contain protons, neutrons and atomic nuclei. In regular matter, these quarks are confined inside protons and neutrons, but in the high density and high-temperature regimes seen in neutron stars, they may move around freely. Scientists must first determine the temperature and density of neutron stars to understand the movement and behaviour of quarks and matter.  
 
OzGrav PhD student and first author Nikhil Sarin says: ‘Our observations show a slight preference for freely-moving quarks. We look forward to getting more observations to definitively solve this puzzle’.
 
The research also found that, before collapsing into black holes, most neutron stars produce faint gravitational waves which are not likely to be individually detected by LIGO.
‘With the construction of more sensitive gravitational-wave detectors, such as the Einstein Telescope in Europe and the Cosmic Explorer in the US, we’re confident that we’ll eventually detect individual gravitational waves from these systems,’ explains Sarin. 

A team of astrophysicists led by PhD student Mike Lau, from the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav), recently predicted that gravitational waves of double neutron stars may be detected by the future space satellite LISA. The results were presented at the 14th annual Australian National Institute for Theoretical Astrophysics (ANITA) science workshop 2020. These measurements may help decipher the life and death of stars.
 
Lau, first author of the paper, compares his team to ‘astro-palaeontologists’: ‘Like learning about a dinosaur from its fossil, we piece together the life of a binary star from their double neutron star fossils.’
 
A neutron star is the remaining ‘corpse’ of a huge star after the supernova explosion that occurs at the end of its life. A double neutron star, a system 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 and made headlines when the LIGO/Virgo Collaboration detected them for the first time in 2015. These gravitational waves formed when a pair of black holes spiralled too close together and merged.
 
However, 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.
 
The recent study shows that the Laser Interferometer Space Antenna (LISA) could potentially detect these gravitational waves from double neutron stars. LISA is a space-borne gravitational-wave detector that is scheduled for launch in 2034, as part of a mission led by the European Space Agency. It’s made of three satellites linked by laser beams, forming a triangle that will orbit the Sun. Passing gravitational waves will stretch and squeeze the 40 million-kilometre laser arms of this triangle. The highly sensitive detector will pick up the slowly-oscillating waves—these are currently undetectable by LIGO and Virgo.
 
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 dozens of double neutron stars as they orbit each other. Their results were published in the Monthly Notices of the Royal Astronomical Society.
 
A supernova explosion ‘kicks’ the neutron star it forms and makes the initial circular orbit oval-shaped. Usually, gravitational wave emission rounds off the orbit—that is the case for double neutron stars detected by LIGO and Virgo. But LISA will be able to detect double neutron stars when they’re still far apart, so it may be possible to catch a glimpse of the oval orbit.
 
How oval the orbit is, or the eccentricity of the orbit, can tell us a lot about what the two stars looked like before they became double neutron stars. For example, their separation and how strongly they were ‘kicked’ by the supernova.
 
Our 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.

Astronomers regularly observe gravitational waves (GW)—ripples in space and time—that are caused by pairs of black holes merging into one. Einstein’s theory of gravity predicts that GW, which squeeze and stretch space as they pass, will permanently distort space, leaving a ‘memory’ of the wave behind. However, this memory effect has not yet been detected as it’s extremely small, leaving the faintest traces.

Researchers from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) at Monash University have finally developed a method to search and detect GW memory. Led by OzGrav PhD student Moritz Huebner, the recently published paper explains the tricky conquest of searching for memory by analysing data from numerous different observations. Huebner will be presenting these results at the Australian National Institute for Theoretical Astrophysics (ANITA) in Canberra this Thursday 6 February 2020.
 
The scientific models expect memory to leave an extremely faint trace on the detectors which is far smaller than the waves from the black hole collision itself.  Therefore, data needs to be combined from many different gravitational wave events. To do this, the team used some of the most precise GW and memory models developed from the study of black hole mergers.
 
‘Our algorithms carefully comb through the data and measure the exact evidence for the existence of GW memory,’ said Huebner.
 
For each individual observation, this painstaking method can take hundreds of hours on a normal computer chip to explore all the possibilities of how a GW signal came about—this prompted the researchers to focus on fine-tuning the setting to reduce the amount of computing hours without compromising the search. So far, the results of the search applied to the first ten black-hole collisions—detected by LIGO and Virgo between 2015 and 2017—have proven inconclusive. LIGO and Virgo are not yet sensitive enough to make any statements about GW memory.


So, will we ever be able to detect memory?
 
‘Thankfully, we can now use data from the first ten black-hole collisions and have a decent idea of how many observable GW events there will be in the future. We can also calculate how much evidence of memory can be detected in each event,’ said Huebner.
 
Throughout the study, the researchers also discovered that their new search method must take data from approximately 2000 black hole mergers to detect memory. While this might sound implausible, the team expects to hit this number by the mid-2020s.
 
Plus, LIGO and Virgo are continuously being upgraded and have seen more than 40 mergers since April 2019, when the third observation run started. With further technological advances and the Japanese KAGRA observatory soon coming online, the team is confident that they’ll detect multiple binaries every day which will finally lead to revealing GW memory.
 
Link to journal: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.101.023011
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Scientists from the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav) reveal the eccentricity of binary black holes: the shape of the orbit formed when two black holes fall into a dance as they spiral towards each other and eventually collide. While the most common orbit is thought to be circular, about one in 20 are in egg-shaped eccentric orbits, which can indicate completely different binary life histories.

Since the first detection of gravitational waves (GW) in September 2015, LIGO and its European counterpart Virgo have published the discovery of ten merging black-hole binaries. The latest run has already uncovered more than 30 new detections, with more forecast by April 2020.

OzGrav PhD student (and first author) Isobel Romero-Shaw recently published a study on the origins of GW 190425 – an event which was only announced this month (January 2020) by the LIGO/Virgo collaboration.

The GW signals provide a wealth of information about the pre-merger binaries; however, no one has yet deciphered how these black holes pair up in the first place.

New research,  published in the journal Monthly Notices of the Royal Astronomical Society, reveals an important clue to how these black hole binaries are formed, how long they’ve been ‘together’ and what happens when they finally collide.

The study, led by Romero-Shaw, OzGrav Chief Investigator Eric Thrane and Associate Investigator Paul Lasky—all from Monash University—looked at data from the first and second rounds of observation of LIGO and Virgo, in particular, the ten black hole collisions that these two observation runs confirmed. They found that the orbits of all ten of these systems were remarkably circular, which is consistent with the expectation that about one in 20 orbits are not.

The current LIGO/Virgo run has already detected more than 30 additional collision signals. According to Romero-Shaw, the large amount of data coming from the third observing run ‘will mean we are much more likely to see eccentric collisions of black holes, which will give us real insight into how these systems form’.

According to Thrane, the more common circular orbits come from black holes who have been together from when they were garden-variety stars before they exploded and became black holes. Thrane explains: ‘These binaries are like siblings if you like. They grew up together and their orbit is circular’.

Eccentric orbits occur when black holes fall under each other’s gravitational influence by chance as they are zipping around galaxies. ‘These are more like adults who meet later in life and pair up.  Their orbital relationship is more interesting -- much like in life,’ he added.

Importantly, when these two objects collide, the shape of their orbit means their gravitational-wave signal looks different. These detected explosions can now be used to retrospectively study the objects that collided.

Lasky said that the current LIGO and Virgo observing run is detecting ‘large numbers of these binaries and by April 2020—when the run finishes—we will have a far greater insight into what these events mean’.
 
VIDEO EXPLAINER: https://www.youtube.com/watch?v=LmW4Hd4sJvg
Credit: Johan Samsing