14 mins read 31 Jan 2020

Pulsar – White Dwarf system provides new evidence of ‘Frame-Dragging’

A new journal article published in Science today has found further supporting evidence for Einstein’s General Relativity through an unusual binary pair of exotic stellar objects.

In August 1997, the Parkes Radio Telescope began sweeping its observing beams across the sky – primarily designed to catch cold hydrogen gas lurking across the Galaxy. Earlier that year the iconic telescope, having played such an important historical role, was upgraded to include a new filterbank system able to take in the data from 13 beams that gazed upon the heavens, speeding up the time it took to survey the sky. The sensitivity of the telescope, combined with its new faster-scanning capabilities made it an ideal instrument in the science and discovery, of the Universe’s natural, yet extreme, gravitational laboratories – pulsars.

The outpouring of data from the study which ran for a number of years since the new receiver was installed, known as the Parkes Multibeam Pulsar Survey, would discover hundreds of new pulsars and revolutionise our understanding of these remarkable objects – giving off their radio pulses from the distant corners of our Galaxy.

Around the year 2000, one such pulsar (named: PSR J1141-6545) was found to have a peculiarly heavy companion, forming a binary system, of the order of a White Dwarf star. As the orbital dance of these exotic stars flowed around a common centre of gravity within the system – the long-axis of the orbit would shift by at 5.5 degrees per year (this is known as orbital precession).

At the time, the orbital precession of PSR J1141-6545 was the highest known value, and the only way to explain this would be to go back to Einstein’s description of gravity – written some 85 years earlier.  

Since its discovery, PSR J1141-6545 has thrilled scientists as it continues to provide the opportunity to test Einstein’s Theory of General Relativity. After tracking the exotic stellar pair for 20 years using the Parkes Radio Telescope, an international team of astrophysicists led by Australian Professor Matthew Bailes, from the ARC Centre of Excellence of Gravitational Wave Discovery (OzGrav), has shown exciting new evidence for ‘frame-dragging’— one of the effects of how space-time behaves according to General Relativity. The findings have been published today in the prestigious journal, Science.

The result is especially pleasing for team members Prof. Matthew Bailes, Willem van Straten (Auckland University of Tech) and Ramesh Bhat (ICRAR-Curtin) who have been trekking out to the Parkes 64m telescope since the early 2000s, patiently mapping the orbit of this wild exotic pair with the ultimate aim of studying Einstein’s Universe. “This makes all the late nights and early mornings worthwhile”, said Bhat.

Australia and New Zealand form part of International Collaboration

A number of Australian and New Zealand institutions have been involved in the international collaboration and the release of this new paper, including Swinburne University of Technology, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), International Centre for Radio Astronomy Research (ICRAR) and Auckland University of Technology.

International collaborators included members from the Max Planck Institute of Radio Astronomy in Germany, the Square Kilometer Array organisation in the United Kingdom, and Aarhus University in Denmark.

The Parkes Radio Telescope

The Parkes Dish photographed at night looking skyward. The bright moon is behind it.
The Parkes Radio Telescope. Credit: Shaun Amy/CSIRO.

The Parkes Radio Telescope has played an undeniably important role in the history of pulsar astronomy since it began its operations in 1969. The radio-wavelength antenna, a staggering 64m in diameter – and the many astronomers who have worked with it – have all contributed to the global knowledge bank of exotic objects such as pulsar.

Big enough to fit the length of an Olympic pool (with seven meters to spare on either side), the telescope has also contributed to space-exploration missions such as the Mariner missions to the terrestrial planets, communicating with the Galileo and Cassini spacecraft as they orbited the Gas Giants, and kept in contact with the Voyager missions – spacecraft who’ve now left our Solar System and traverse alone as the furthest human-built objects, heading for the stars.

Some of the first television signals and data from humanity’s first mission to land on the Moon, Apollo 11, were acquired by Parkes before beaming out to the rest of the world.

More recently, the Parkes Telescope – colloquially known as ‘The Dish’ (after a popular Australian film), has once again pushed the boundaries of science – assisting in the search for mysterious extragalactic radio transient pulses known as Fast Radio Bursts (FRBs). In fact, the first FRB ever detected in 2007 was found in the archival data from a 2001 observation recorded by the Parkes telescope.

When a star dies, a pulsar is born

Diagram showing star births on left through to main sequence then a division of their old age and death.
The life cycle of different mass stars. Credit: R. N. Bailey/Wikipedia

The system that pulsar J1141-6545 belongs to evolved a rather interesting, yet violent-filled history. It wasn’t always a pulsar and a white dwarf, originally there would have been two stars – locked as a binary pair – much like most stars in our Universe.

How a star ends the portion of its life, known as the main sequence phase, all depends on how big the star originally was. The main sequence phase of any star is when it is fusing hydrogen in its core to produce energy. The larger the original star, the more greedily and quickly it consumes its hydrogen fuel source.

Stars like our Sun will eventually burn through their hydrogen in about 10 billion years. Our local star is currently about halfway through this main sequence part of its life. But when a star as massive as our Sun comes to the end of its main-sequence stage, it starts to burn newly created elements – like Helium, Carbon, and Oxygen (these are created through the nuclear fusion processes in its core).

The fusion of these new elements into heavier ones, causes the outer layers of stars like the Sun to expand and puff off, eventually leaving nothing more than a white-hot core, about the size of a planet like Earth. This is known as a ‘white dwarf’ star.

However, when the original star is many times more massive than our Sun – it ends in a violent supernova explosion – one of the most cataclysmic events in our universe. The supernova event, triggered by the enormous gravity of the original star creates a more extreme object as a remnant, relative to a white dwarf.

When the star is massive enough, an extremely dense neutron star will form – a single teaspoon of its material would weigh as much as a cube of Earth with edges measuring 800m (so 512,000 cubic km). Neutron stars usually are about the size of a city, measuring about 20km in diameter. The colossal mass squeezed into such a small space creates a powerful gravitational field, where it would require an acceleration of 150,000km/s to lift you off the surface – should you survive being crushed into pure energy.

Neutron stars also have immense magnetic fields and rotate rapidly – some hundreds of times per second. When combining these two factors, some neutron stars emit beams of radio energy from their magnetic poles – beams that sweep past the Earth with each rotation and are detected as ‘pulses’ by telescopes on Earth. These are known as ‘pulsars’.

Lastly, if the original star is so massive – nothing can stop its collapse during the supernova event – and the star remnant becomes an elusive black hole, an object where not even light can escape.

The Unique System of PSR J1141-6545

Illustration showing the pulsar orbiting the white dwarf. The pulsar beams are warped as is the space-time around the white dwarf
Artist’s depiction of a rapidly spinning neutron star and a white dwarf dragging the fabric of space-time around its orbit. Credit: Mark Myers, OzGrav ARC Centre of Excellence.

What makes this binary system so unique is the coupling of a young neutron star with an old white dwarf companion – especially where the mass of both objects is asymmetrical (that is, one is heavier than the other, an asymmetric binary). This presents an opportunity to test a number of General Relativity postulates in an extreme environment, from the safe distance of Earth.

“After ruling out a range of potential experimental errors, we started to suspect that the interaction between the white dwarf and neutron star was not as simple as had been assumed to date”, said Willem van Straten from Auckland University of Technology.

Before the more massive star detonated in a supernova (thus resulting in a pulsar), a million or so years ago, it began to swell up discarding its outer core which fell onto the white dwarf nearby. This falling debris made the white dwarf spin faster and faster until its day was only measured in terms of minutes. It’s this history that makes this system fascinating to astrophysicists.

Due to the extremely accurate regularity and measurements of observing pulsar signals (the pulses), astrophysicists are able to derive so much information about the pair of extreme objects, orbiting far off in our Galaxy. For the systems normal parameters (Keplerian):

  • Pulsar diameter: ~20 km across
  • Pulsar mass: 1.27 times the mass of the Sun
  • Pulsar spin rate: Full rotation on its axis once every 394 milliseconds
  • Pulsar spin rate slowdown (P-Dot): -2.767986 x 10-14
  • Pulsar density: approx. 100 billion times that of the Earth
  • White dwarf diameter: ~ 12,700km across
  • White dwarf mass: 1.02 times the mass of the Sun
  • White dwarf density: 300,000 times that of the Earth
  • Total mass of system: 2.2892 times the mass of the Sun
  • Orbital period: 0.197650 days or ~4.74 hours
  • Orbital eccentricity (e): 0.1718753
  • Orbital inclination angle: 76 degrees relative to the plane of our sky
  • Distance from Earth: 3.7 kiloparsecs or just over 12,000 light-years
  • Dispersion measure: 116.080 pc cm-3

As noted in the data above (obtained in this paper) both stars are on the extreme side of our everyday perceptions and familiarities, which make it the perfect environment to test a 100-year old theory that has withstood numerous assessments and continued to be celebrated as the best model scientists have to explain the dominant force, known as gravity.

Science Check: What is General Relativity?

Illustration of Earth, surrounding by grid lines which are warped towards it
Massive objects, like the Earth, cause surrounding space-time to warp. Credit: medium.com.

Developed and published in 1915 by Albert Einstein, General Relativity (also known as GR; Theory of General Relativity) revolutionised the understanding of gravity as a force.

Prior to GR, Newtown’s laws of universal gravitation applied across all branches of physics – but as observational data began to grow, Newton’s laws were lacking at being able to explain how gravity worked across the universe in certain situations. As an example, scientists questioned why the major axis in Mercury’s orbit around the Sun, would slightly shift every year (this is known as precession). Newton’s laws could not explain this.

When Einstein came along, GR changed the concepts of space and time (into a singular space-time) and defined that gravity was a result of the curvature of space-time, caused by the presence of matter and energy. In a nutshell, the more massive an object – the more curvature it will produce of the localised space-time surrounding it. This is interpreted as gravity.

The gravity of the Earth, the speed at which aircraft move, the orbital dance of the planets – these all continue to follow the rules as set out by Newton. However, GR strays away from our concepts of normality and into the extremes of the very massive, the very fast, the very dense, the very big.

Some differences that GR contrasts against classical physics are:

  • Gravitational redshift of light – as photons of light try to climb out of a gravitational field, their wavelengths are stretched proportionally by the strength of the gravitational field. They are said to be redshifted
  • Gravitational time delay (also known as Shapiro Delay) – as signals pass near a massive object, localised space-time is curved enough to cause an observable delay in the signal propagating through this space-time and take longer to travel to their target
  • Gravitational lensing – when a massive object lies within the line of sight of the observer and a light source behind it, the massive object will bend the emitted light from the source around the massive object as it travels towards the observer
  • Gravitational time dilation – the measured difference in time that has elapsed between two events that occur at different distances from a gravitational wave source
  • Rotational Frame-dragging – when massive objects rotate, they drag the fabric of space-time with them in the direction of rotation
  • Gravitational radiation (also known as gravitational waves) – propagating waves that radiate in space-time at the speed of light as a result of accelerating masses
  • Orbital precession – a shift in the orientation of the long axis of an ellipses orbit, caused by the curvature of space-time near a massive body

GR in itself is still not perfect, with many scientists working towards finding how it could be integrated with quantum physics, to quantify gravity on the very small scale. However, since its release, all predictions made by GR have been confirmed in experiments to date.

Outline in this November 2019 paper, the PSR J1141-6545 system was observed to have the following Post-Keplerian parameters – the results from GR tests:

  • The System was found to have Time dilation/Gravitational redshift: 0.000773 milliseconds
  • The orbital precession of the system was found to shift by 5.3096 degrees per year
  • The orbital period shrunk by -0.403 x 10-12 per orbit as the system was found to be radiating gravitational energy (and the two objects are predicted to merge in several hundred million years)

Space-Time Frame-Dragging

Illustration of the Earth rotating and dragging an imaginary grid around with it into a spiral.
Artist illustration of the rotating Earth, frame-dragging surrounding space-time as it rotates. Credit: Stanford University.

One of the most exciting finds from the journal article released today in Science is that the PSR J1141-6545 system has produced new evidence of frame-dragging – known as the ‘Lense-Thirring effect’, further supporting Einstein’s GR.

This concept of frame-dragging began in 1918 when Austrian mathematicians Josef Lense and Hans Thirring realised that if Einstein was right all rotating bodies should ‘drag’ the very fabric of space-time around with them. In everyday life, the effect is minuscule and almost undetectable.

However, the astrophysicists who authored this new paper realised that frame-dragging of the entire orbit could explain the tilting orbits observed within the PSR J1141-6545 system, indicating further compelling evidence in support of GR being alive and well, exhibiting yet another one of the theory’s many predictions.

“We postulated that this might be, at least in part, due to the so-called “frame-dragging” that all matter is subject to in the presence of a rotating body as predicted by the Austrian mathematicians Lense and Thirring in 1918” said Dr. Paulo Freire, one of the international collaborators from the Max Planck Institute for Radio Astronomy in Germany.

Further experimental evidence of frame-dragging was observed in gyroscopes orbiting the Earth, whose orientation was dragged in the direction of the Earth’s spin. A rapidly spinning white dwarf, like the one in PSR J1141-6545, drags space-time 100 million times as strong as the Earth – allowing the pulsar’s signal to be analysed accurately.

The new results from the PSR J114-6545 system now join a chorus of experimental data that supports Einstein’s General Relativity – in addition to highlighting the important contribution to the sciences that the now 51-year old Parkes Radio Telescope continues to provide.

The uniqueness of the stellar system – it’s asymmetry in particular – have proven advantages in continually gathering data over the long-term, repetitively testing observations and refining results that tell us more about what remains of stars after their main sequence phase has ended, and how extreme objects like pulsars bend the very structure of space-time.

It must be exciting to see such beautiful results for some of these scientists, who’ve been traveling out to the Parkes Radio Telescope for two decades – patiently spending days and nights gathering data from the pulsar, as its beams rapidly swept past the Earth.

It must also raise may many more questions about General Relativity, ideas of experiments to perform, and theories to draw upon and define the reality we exist in.

The only constant in this story is the tenacious ticking of the pulsar out in the Galaxy. Like a clock with an everlasting battery, one ponders if its pulses will continue to reveal more of the mysteries of our Universe or change our perception of it?

Only time will tell.

The journal article appears in the latest issue of Science