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If the conditions are right, the star will continue to collapse in on itself until what’s left is a remnant of what was there before, usually only about the size of the Melbourne CBD, but 1-2 times as heavy as our Sun, making these some of the densest objects in the Universe.

These stars don’t produce much visible light, but from their magnetic poles, they emit surprisingly bright beams of radio waves. If we’re lucky, as the star rotates, those beams will wash over the Earth and we observe ‘pulses’. While most pulsars spin around in about a second, there is a subclass of these stars that spin around in just a few thousandths of a second—they’re called ‘millisecond’ pulsars.

Observing the pulses from these millisecond pulsars gives physicists clues to many questions, including testing General Relativity and understanding the densest states of matter. But one of the main goals of observing these incredibly fast, dense stars is to detect ultra-long wavelength gravitational waves. And by long, we mean many light-years long. These gravitational waves distort space-time between us and the pulsars, causing the pulses to arrive earlier or later than expected. It’s likely that these gravitational waves come from a background produced by all the binary supermassive black holes in the Universe, which form from galaxies crashing into one another.

As part of OzGrav, we try and detect this gravitational wave background by looking at collections of the most predictable stars (called pulsar timing arrays) and measuring how they change over time. We did this by using the world’s most sensitive radio telescopes, including the Australian Murriyang telescope (also known as the Parkes telescope) and the ultra-sensitive MeerKAT array telescope in South Africa.

But it’s not quite that simple. From our observations with MeerKAT we found that the most precisely timed (read: predictable) pulsar, J1909-3744, was misbehaving. We found that the pulses were changing shape, with bright pulses arriving earlier and narrower than faint ones. This lead to greater uncertainty in its predicted emission. Fortunately, we were able to establish a method to account for this change and time tag the pulsar more precisely than ever before. This method could be of use for other pulsars and will be important when more advanced telescopes are available in the future.

​Written by OzGrav PhD student Matthew Miles, Swinburne University

Key points:

• A new international study, led by an Australian researcher from the ARC Centre of Excellence for Gravitational Wave Discovery, searched for elusive continuous gravitational waves from the densest objects in the Universe–neutron stars.
• A detection of a continuous gravitational wave would allow scientists to peer into the hearts of these neutron stars–they are extremely dense, collapsed cores of massive supergiant stars.
• The hunt for continuous gravitational waves is one of the top challenges in gravitational wave science, but Australia has a strong track record in this area of research.

​Take a star similar in size to the Sun, squash it down to a ball about twenty kilometres across and you’d get a neutron star: the densest object in the known Universe. Now set your neutron star spinning at hundreds of revolutions per second and listen carefully. If your neutron star isn’t perfectly spherical, it will wobble a bit, causing it to continuously send out faint ripples in the fabric of space and time. These ripples are called continuous gravitational waves.
So far, these elusive continuous gravitational waves haven’t been detected; however, in a recent study, an international collaboration of scientists, led by Australian OzGrav researcher Julian Carlin (from the University of Melbourne), searched for them from a specific category of neutron star: accreting millisecond X-ray pulsars (AMXPs).

To break it down, AXMPs are:

• Pulsars ━ The Universe’s lighthouses; they are extremely dense collapsed cores of massive supergiant stars (called neutron stars) that beam out radio waves, like a lighthouse. As a pulsar rotates, we can see a pulse in radio telescopes every time the beam points towards the Earth.
• Accreting pulsars ━  they have a companion star and this is called a binary star system. The accreting pulsar feeds off its companion star, sucking up matter from the star and accumulating it on their surface.
• X-ray pulsars ━ they emit X-ray pulses. AMXPs have times of “outburst” where the X-ray pulses are observable and times of “quiescence” when X-ray pulses are either not emitted or are too weak to see.
• Millisecond pulsars━ they spin very fast (a millisecond is one thousandth of a second). The fastest spinning AMXP takes just 1.7 milliseconds to do a full rotation. That means if you were standing on the surface you would be whipping around at 15% the speed of light (or about 45,000 km/s)!

As AMXPs accumulate matter from their companion star, they’re likely to send out stronger signals than a lone neutron star. This is because the strength of a neutron star’s signal is proportional to its asymmetry. Astronomers theorise that this build up of matter on the AMXPs could create small mountains on the surface as material is funnelled by the magnetic field onto the magnetic poles. This is illustrated by the artist’s impression shown in Figure 1.

This search uses data from the third observing run of LIGO, Virgo, and KAGRA which lasted from April 2019 to March 2020. The team searched for continuous gravitational waves from 20 AMXPs – 14 of which hadn’t been searched before.

The search method used in this work is the result of a collaboration between physicists and engineers at the University of Melbourne. “The methods we are using to search for continuous gravitational waves from spinning neutron stars are similar to those used in speech recognition software!” said Hannah Middleton (an OzGrav postdoc at both the University of Melbourne and Swinburne University).

Unfortunately, continuous gravitational waves were not convincingly detected this time. However, as detector technology and data analysis algorithms keep improving, it’s possible that a detection will be made in the next observing run.

Julian Carlin said: “It may turn out that the weak candidates we’ve spotted here are the first signs of a real signal, and we just need a little bit more data to pull it out of the noise”.

“If a detection were made, it’d allow us to peer into the hearts of neutron stars ━ teaching us how matter behaves in extremely dense environments,” he continues. “Detecting continuous gravitational waves from neutron stars would give us great insights into how these fantastic astronomical clocks really tick.”

“The hunt for continuous gravitational waves is one of the top challenges in gravitational wave science”, said Andrew Melatos, an OzGrav Chief Investigator whose research group at the University of Melbourne has been chasing these tiny signals for more than a decade. “Pulsars are one of Nature’s most bountiful gifts. Their radio signals revolutionised astronomy, shedding new light on everything from the gas between the stars to Einstein’s theory of gravity and the strongest magnetic fields in the Universe. Who knows what surprises their gravitational wave murmurs will reveal?”

Dr. Karl Wette, an OzGrav research fellow at The Australian National University and co-chair of the LIGO continuous wave working group, said: “Gravitational waves are becoming an essential tool for fundamental physics and astronomy. We’ve now heard the echoes of nearly 100 pairs of black holes and neutron stars smashing into each other. We’re keeping our ear to the ground, and hope to pick out the tell-tale hum of a rapidly-spinning neutron star in the coming years. Australia has a strong track record in this area of research, and it’s particularly pleasing to see Australian students and junior researchers making important contributions.”
“With improved detectors in the fourth observation run, the number of detections is expected to increase manifold,” said OzGrav PhD student Chayan Chatterjee at the University of Western Australia. “So, it will be extremely exciting to watch out for more continuous gravitational wave candidates as well as other ground-breaking discoveries!”

Read the full scientific article https://journals.aps.org/prd/abstract/10.1103/PhysRevD.105.022002
Link to the LIGO Science Summary: https://www.ligo.org/science/Publication-O3LMXBsAMXPs/.

​So, is there something we’re missing? In our paper we show that the puzzling black hole spin is probably due to something that was added to the LIGO-Virgo measurements instead. It has to do with things called ‘priors’ which encode assumptions about the population of black hole-neutron star binaries based on our current knowledge. We argue that a better explanation for the GW200115 merger is that the black hole was not spinning at all, and consequently, we place tighter constraints on the black hole and neutron star masses.

What is a prior?
Imagine you want to know the probability of having drawn an Ace from a deck of cards, given that the card is red. You’d need to know the separate probabilities of drawing an Ace and a red card. The probability of drawing an Ace, independent of the data (“the card is red”) is the ‘prior’ probability of drawing an Ace. Astronomy is similar to a game of cards: we can think of observed gravitational-wave signals as having been dealt to us randomly by the Universe from a cosmic deck of cards. The prior should express our current best knowledge of this deck before we make a measurement, because it‘s used to calculate the probability of each possible black hole spin. In the LIGO-Virgo analysis of GW200115, it was assumed that all black hole spins are equally likely. This is fine if we have no strong preference for any value, but we do: observation and theory tell us we shouldn’t expect a rapidly spinning black hole to be paired with a neutron star. This information is key to accurately measuring the properties of  GW200115.

In our paper, we begin by demonstrating that if GW200115 originated from a black hole-neutron star binary with zero spin, the unrealistic LIGO-Virgo prior (which assumes the black hole can equally likely spin with any magnitude and direction) generates preference for a large misaligned black hole spin. We do this by simulating a gravitational-wave signal from a non-spinning binary, placing it into simulated (but realistic) LIGO-Virgo noise, and inferring its properties assuming any spin value is equally likely. Our simulated experiment yields a similar spin measurement to LIGO-Virgo’s and we’re able to explain analytically why signals from black hole-neutron star binaries with zero spin will generically yield such measurements when very broad spin priors are assumed. While this doesn’t prove that GW200115 is non-spinning, it suggests that the puzzling LIGO-Virgo spin measurement is probably due to their unrealistic priors.

Next, we look to astrophysics to figure out a more realistic prior. We use current theoretical modelling to suggest that there’s roughly a 95% probability that black hole-neutron star binaries do not spin at all, and only around 5% do spin. We use this astrophysical prior to update the LIGO-Virgo measurements of GW200115’s spins and masses. When we do this, we find that there is almost zero probability that the black hole had any spin at all. While this might seem circular at first glance—after all, we’re giving zero-spin almost 20 times more weight than non-zero spin—it’s also a reflection of the fact that the data don’t strongly support a rapidly spinning black hole. Additionally, we show that our prior reduces the uncertainty on the black hole and neutron star masses by a factor of 3. Reassuringly, the mass of the neutron star looks significantly more like those found in double neutron star systems in the Milky Way.

Written by Rory Smith and Ilya Mandel, Monash University

This field gained enormous recognition after the joint detection of gravitational waves and gamma ray bursts in 2017. Gravitational waves can be used to identify the sky direction of an event in space and alert conventional telescopes to follow-up for other sources of radiation. However, following up on prompt emissions would require a rapid and accurate localisation of such events, which will be key for joint observations in the future.

The conventional method to accurately estimate the sky direction of gravitational waves is tedious—taking a few hours to days—while a faster online version needs only seconds. There is an emerging capacity from the LIGO-Virgo collaboration to detect gravitational waves from electromagnetic-bright binary coalescences, tens of seconds before their final merger, and provide alerts across the world. The goal is to coordinate prompt follow up observations with other telescopes around the globe to capture potential electromagnetic flashes within minutes from the mergers of two neutron stars, or a neutron star with a black hole—this was not possible before. The University of Western Australia’s SPIIR team is one of the world leaders in this area of research. Determining sky directions within seconds of a merger event is crucial,as most telescopes need to know where to point in the sky. In our recently accepted paper [1], led by three visiting students (undergraduate and Masters by research) at the OzGrav-UWA node, we applied analytical approximations to greatly reduce the computational time of the conventional localisation method while maintaining its accuracy. A similar semi-analytical approach has also been published in another recent study [2].

The results from this work show great potential and will be integrated into the SPIIR online pipeline going forward in the next observing run. We hope that this work complements other methods from the LIGO-Virgo collaboration and that it will be part of some exciting discoveries.

Written by OzGrav PhD student Manoj Kovalam, University of Western Australia.

[1] https://doi.org/10.1103/PhysRevD.104.104008
[2] https://doi.org/10.1103/PhysRevD.103.043011
This work is now accepted by PRD: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.104.104008