Humans have been studying the light from stars since the beginning of our history; however, we’ve only just discovered in the last few decades that stars don’t like to be alone.
 
Binary systems—containing two stars orbiting around each other—are one of the most common type of gravitationally-bound collections of stars, yet their evolution is complex. Astronomers are trying to piece together the puzzle of different stellar observations to reveal the bigger picture. Using their understanding of binary evolution, scientists can simulate populations of stellar binaries with the stellar population synthesis code COMPAS—mostly developed by researchers from the ARC Centre of Gravitational Wave Discovery (OzGrav).
 
OzGrav researchers, in collaboration with the Max Planck Institute of Hannover, Monash University and University of Birmingham, recently conducted a study to understand the origin of the properties of ‘Be X-ray’ binaries observed in the Small Magellanic Cloud.

Be X-ray binaries are star systems typically composed of a neutron star orbiting around a rapidly rotating massive star. This rotation causes the massive star to produce a disk of outflowing material—some of this is accumulated by the neutron star. The neutron star then shoots off X-ray radiation that scientists can observe and measure.

The study, led by OzGrav Affiliate Serena Vinciguerra, used the COMPAS code to simulate an environment like the Small Magellanic Cloud. By comparing the orbital properties of the simulated Be X-ray binaries with the observed ones, researchers revealed the probable evolution of these star systems:​

Initially, two stars are born in a tight binary system. The most massive star evolves quicker and expands. Because of the proximity between the two stars, the inflated massive star feeds its material to the smaller star. Over time, the massive star may feed and lose most of its mass; however, the smaller star may get too ‘full’ and not accept all the ‘food’ (material).

Each star’s individual ‘diet’ depends not only on their constitution and age, but also on the massive star feeding them. In Be X-ray binaries, the stars’ diets are more generous than what astronomers previously assumed. Consequently, the well-fed stars become massive and spin rapidly. ​

Later in their evolution, the original most massive star may explode as a supernova, leaving behind a small but very dense neutron star. If the stars survive the explosion, they form a Be X-ray system, with a neutron star orbiting a massive and rapidly rotating star.

​Researchers have developed a new type of deformable mirror that could increase the sensitivity of ground-based gravitational wave detectors such as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO). Advanced LIGO measures faint ripples in space time called gravitational waves, which are caused by distant events such as collisions between black holes or neutron stars.
 
“In addition to improving today’s gravitational wave detectors, these new mirrors will also be useful for increasing sensitivity in next generation detectors and allow detection of new sources of gravitational waves,” said research team leader Huy Tuong Cao from the University of Adelaide node of the Australian Research Centre of Excellence for Gravitational Waves Discovery (OzGrav).
 
Deformable mirrors, which are used to shape and control laser light, have a surface made of tiny mirrors that can each be moved, or actuated, to change the overall shape of the mirror. As detailed in The Optical Society’s (OSA) journal Applied Optics, Cao and colleagues have, for the first time, made a deformable mirror based on the bimetallic effect in which a temperature change is used to achieve mechanical displacement.
 
“Our new mirror provides a large actuation range with great precision,” said Cao. “The simplicity of the design means it can turn commercially available optics into a deformable mirror without any complicated or expensive equipment. This makes it useful for any system where precise control of beam shape is crucial.”
 
The new technology was conceived by Cao and Aidan Brooks of LIGO as part of a visitor program between the University of Adelaide and LIGO Laboratory, funded by the Australian Research Council and National Science Foundation.
 
Building a better mirror
Ground-based gravitational wave detectors use laser light traveling back and forth down an interferometer’s two arms to monitor the distance between mirrors at each arm’s end. Gravitational waves cause a slight but detectable variation in the distance between the mirrors. 
Detecting this tiny change requires extremely precise laser beam steering and shaping, which is accomplished with a deformable mirror. 
 
“We are reaching a point where the precision needed to improve the sensitivity of gravitational wave detectors is beyond what can be accomplished with the fabrication techniques used to make deformable mirrors,” said Cao.
 
Most deformable mirrors use thin mirrors to induce large amount of actuation, but these thin mirrors can produce undesirable scattering because they are hard to polish. The researchers designed a new type of deformable mirror using the bimetallic effect by attaching a piece of metal to a glass mirror. When the two are heated together the metal expands more than the glass, causing the mirror to bend.
 
The new design not only creates a large amount of precise actuation but is also compact and requires minimum modifications to existing systems. Both the fused silica mirrors and aluminum plates used to create the deformable mirror are commercially available. To attach the two layers, the researchers carefully selected a bonding adhesive that would maximize actuation.
 
“Importantly, the new design has fewer optical surfaces for the laser beam to travel through, said Cao. “This reduces light loss caused by scattering or absorption of coatings.”
 
Precision characterization
Creating a highly precise mirror requires precision characterization techniques. The researchers developed and built a highly sensitive Hartmann wave front sensor to measure how the mirror’s deformations changed the shape of laser light.
 
“This sensor was crucial to our experiment and is also used in gravitational detectors to measure minute changes in the core optics of the interferometer,” said Cao. “We used it to characterize the performance of our mirrors and found that the mirrors were highly stable and have a very linear response to changes in temperature.”
 
The tests also showed that the adhesive is the main limiting factor for the mirrors’ actuation range. The researchers are currently working to overcome the limitation caused by the adhesive and will perform more tests to verify compatibility before incorporating the mirrors into Advanced LIGO.

Pulsars—a type of rotating neutron star—are well-known for their use as incredibly stable astrophysical clocks. Their regularity, used to measure their radio pulses, has led to some of the most exciting tests of Einstein’s general theory of relativity and allowed scientists to examine the behaviour of the extremely dense matter inside neutron stars.
 
But just like ordinary clocks here on Earth, pulsars are not perfect keepers of time. Much like how a watch loses track of a few seconds each year, the exact rate at which pulsars spin appear to randomly wander by tiny amounts over month- to decade-long timescales.
 
The spins of a small fraction of pulsars have also been seen to rapidly speed up—they start ‘ticking’ slightly faster than usual. These effects, called ‘spin noise’ and ‘glitches’, change from pulsar to pulsar and may tell us how neutron stars evolved over millions of years; however, this requires precision tracking of hundreds of pulsar spins over many years.
 
Thanks to a series of upgrades over the last decade, the Molonglo Telescope—which celebrated its 50th birthday in 2015—can perform spin-tracking observations of hundreds of pulsars every two weeks! This enabled researchers, from the ARC Centre of Gravitational Wave Discovery (OzGrav), to find three new glitch events and measure the strength of the spin noise in 300 pulsars. 

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In a recently published study, led by OzGrav PhD student Marcus Lower, researchers examined 280 pulsars that are most representative of normal pulsar evolution and developed a statistical method similar to the one used for analysing gravitational-wave events detected by LIGO and Virgo. The results, presented at CSIRO’s Australia Telescope National Facility colloquium, showed that spin noise seems to decrease with pulsar age and that there is a scaling relationship between spin noise strength, how quickly a pulsar spins and how fast its spin is slowing down over time.

Marcus explains: ‘As spin noise becomes more obvious the longer you stare at a pulsar, we may be able to add additional pulsars to a re-analysis of the Molonglo data set in the future. We can also apply the statistical method to data from telescopes that have been tracking pulsar spins over multiple decades’.
 
The combination of additional pulsars and longer data sets would improve the study’s current measurements and allow researchers to determine the exact cause of spin noise in pulsars.