Gravitational waves are ripples in space-time that come in many forms. So far, short-duration gravitational wave signals have been observed from colliding black holes and colliding neutron stars, but scientists expect to find other kinds of gravitational waves. Recently published research led by the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)studied continuous waves: long-lasting gravitational waves, in this particular case, waves from neutron stars--old dead stars--in specific star systems called low-mass X-ray binaries. Gravitational-wave detectors LIGO (Laser Interferometer Gravitational-wave Observatory) and Virgo provide excellent data to search for continuous waves as their signals are likely to be present in the detector data all the time (compared to gravitational waves from colliding black holes, which last only a second or so).
Neutron stars, which are typically about one and half times the mass of our Sun, are very compact at only 20km across. Some neutron stars are alone, while others are in “binary systems”--the neutron star and a companion star orbit around each other. The OzGrav team focused on looking for continuous waves from spinning neutron stars in "low mass X-ray binaries" (LMXBs). Low mass describes the neutron star's companion which typically has a lower mass than our Sun;they are called X-ray binaries because scientists have observed X-rays from them using X-ray telescopes.
In the study, the team searched for continuous waves from spinning neutron stars by directly targeting five LMXBs, which is a first for these five LMXBs. All the targeted LMXBs have X-ray observations which indicate how fast the neutron star is spinning: its rotation frequency. This is extremely useful information when searching for continuous waves as it’s expected that the frequency of the continuous wave is related to the rotation frequency of the neutron star. This allowed the team to search for each LMXB within a specific frequency range.
Lead author and OzGrav researcher from the University of Melbourne Hannah Middleton says: “We used a search method, developed by researchers at the University of Melbourne,which was previously used to search for another LMXB called Scorpius X-1. Scorpius X-1 is a promising continuous wave source, because its X-rays are very bright, but the X-ray observations were unable to measure Scorpius X-1's rotation frequency. This means that a wide range of frequencies need to be looked at. By taking advantage of the X-ray measurements of rotation frequency for our five LMXBs, we can reduce the computational cost of the search, sometimes by as much as 99 per cent.”
But knowing the rotation frequency is not quite enough:the continuous wave frequency may not equate to the rotation frequency, so the team searched for small frequency ranges around the measured values.
“The continuous wave frequency might even be slowly changing over time, so we need to be able to track it over many months of data,” adds Middleton. “The search uses a technique called a hidden Markov model which is widely used in applications from speech recognition to communication technologies. The resulting search can keep track of a signal even if the frequency changes unpredictably during an observation.”
So, what did the scientists find? After analysing data from the second observing run (over 200 days between November 2016 to August 2017), unfortunately they did not find strong evidence for continuous wave signals from these five LMXBs. But the search continues! LIGO and Virgo's third observation run (from April 2019 to March 2020) has just completed, so the OzGrav scientists have plenty of data analysis and star searching to sink their teeth into.
A cutting-edge vibration sensor may improve the next generation of gravitational-wave detectors to find the tiniest cosmic waves from the background hum of Earth’s motion.
Vibration sensor. Credit: Joris van Heijningen
During his PhD, postdoctoral researcher Joris van Heijningen from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), developed the world’s most sensitive inertial vibration sensor. Now, he proposes a similar design, but 50 times more sensitive, at frequencies below 10 Hz, using cryogenic temperatures.
This new sensor measures vibrations as small as a few femtometre (a millionth of a billionth of a meter) with a 10 to 100 millisecond period (10 Hz to 100 Hz). The paper recently published in IOP’s Journal of Instrumentation reveals a prototype of the next generation of seismic isolation systems with sensitivity down to 1Hz, using cryogenic temperatures—lower than 9.2 degrees and above the absolute zero.
Even though we can’t feel it, our planet is always vibrating a tiny bit due to many different events, both cosmic and earthly; for example, from gravitational waves (miniscule ripples in spacetime); ocean waves crashing on the shore; or human activity. According to Dr van Heijningen, some places vibrate more than others and, if you plot these vibrations, they lie between two lines called the Peterson Low and High Noise Models (LNM/HNM).
‘The best commercial vibration sensors have been developed to have a sensitivity that lies below the LNM. They are sufficiently sensitive to measure all places on Earth with a decent signal-to-noise-ratio,’ says van Heijningen.
To date, the Laser Interferometer Gravitational-Wave Observatory (LIGO), with its four-kilometre long arms, uses seismic isolation systems to prevent earthly vibrations affecting scientific measurements; however, future gravitational wave-detectors demand more advanced and precise vibration sensors.
Scientists are already working on a third generation of detectors that will have the power to detect hundreds of black-hole mergers each year, measuring their masses and spins—even more than LIGO, or its European equivalent, Virgo, can measure.
In the US, there will be the Cosmic Explorer: a 40-kilometre observatory that will be able to detect hundreds of thousands of black-hole mergers each year. Equally as impressive will be the Einstein Telescope in Europe, with its 10-kilometre armed, triangular configuration built underground.
Future detectors will be able to measure gravitational waves at frequencies lower than the current cut-off ~10 Hz, ‘because that’s where the signals from collisions of black holes are lurking,’ van Heijningen explains. But one of the main issues of these huge detectors is that they need to be extremely stable—the smallest vibration can hamper detections.
‘Essentially getting the system close to zero degrees Kelvin (which is 270 degrees below zero celsius) drastically reduces the so-called thermal noise, which is dominant at low frequencies. Temperature is a vibration of atoms in some sense, and this minuscule vibration causes noise in our sensors and detectors,’ says van Heijningen.
Future detectors will need to cool down to cryogenic temperatures, but it is no easy feat. Once scientists achieve that, exploiting the cryogenic environment will improve sensor performance following this proposal design. At his new position as a research scientist at UCLouvain in Belgium, van Heijningen plans to prototype this sensor design and test its performance for The Einstein Telescope.
On March 12th 2020 a space telescope called Swift, detected a burst of radiation from half-way across the Milky Way. Within a week, the newly discovered X-ray source, named Swift J1818.0–1607, was found to be a magnetar: a rare type of slowly rotating neutron star with one of the most powerful magnetic fields in the Universe.
Spinning once every 1.4 seconds, it’s the fastest spinning magnetar known, and possibly one of the youngest neutron stars in the Milky Way. It also emits radio pulses like those seen from pulsars--another type of rotating neutron star in our galaxy. At the time of this detection, only four other radio-pulse-emitting magnetars were known, making Swift J1818.0–1607 an extremely rare discovery.
In a recently published study led by a team of scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), it was found that the pulses from the magnetar become significantly fainter when going from low to high radio frequencies: it has a ‘steep’ radio spectrum. Its radio emission is not only steeper than the four other radio magnetars, but also steeper than ~90% of all pulsars! Additionally, they found the magnetar had become over 10 times brighter in only two weeks.
Comparatively, the other four radio magnetars have almost constant brightness at different radio frequencies. These observations were made using the Ultra Wideband-Low (UWL) receiver system installed on the Parkes radio telescope, also known as ‘The Dish’. Whereas most telescopes are limited to observing radio waves across very narrow frequency strips, the Parkes UWL receiver can detect radio waves across an extremely wide range of frequencies all at the same time.
After further analysis, the OzGrav team found interesting similarities to a highly energetic radio pulsar called PSR J1119–6127. This pulsar underwent a magnetar-like outburst back in 2016, where it too experienced a rapid increase in brightness and developed a steep radio spectrum. If the outburst of this pulsar and Swift J1818.0–1607 share the same power source, then slowly over time, the magnetar’s spectrum should begin to look like other observed radio magnetars.
The age of the young magnetar (between 240-320 years), was measured from both its rotation period and how quickly it slows down over time; however, this is unlikely to be accurate. The spin-down rates of magnetars are highly variable on year-long timescales, particularly after outbursts, and can lead to incorrect age estimates. This is also backed up by the lack of any supernova remnant—remnants of luminous stellar explosions—at the magnetars position.
Lead author of the study Marcus Lower proposed a theory to explain of the magnetar’s mysterious properties: ‘Swift J1818.0–1607 may have started out life as a more ordinary radio pulsar that obtained the rotational properties of a magnetar over time. This can happen if the magnetic and rotational poles of a neutron star rapidly become aligned, or if supernova material fell back onto the neutron star and buried its magnetic field’.
The buried magnetic field would then slowly emerge back to the surface over thousands of years. Continued observations of Swift J1818.0–1607, over many months to years, are needed to test these theories.