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12 mins read 23 Oct 2019

Serendipitous Discovery of a Pulsar using ASKAP

Whilst searching for flares from Earth’s closest stars, scientists discover a rapidly spinning compact stellar remnant known as a pulsar.

In May 2019, Andrew Zic’s time to work with one of Australia’s most powerful astronomical instruments had arrived. As a University of Sydney/CSIRO Ph.D. student, Andrew was planning on using his four-day observation period to study the closest star to our solar system, Proxima Centauri for a better understanding of bright flares from the star.

At the same time, Emil Lenc, a CSIRO astronomer based in Sydney, was pushing the instrument and really stretching its legs to find more efficient ways to process the large volumes of data that are generated when its great eye turns towards the heavens.

It was during this small, four-day window that Emil noticed something strange towards the edge of the image that the instrument produced. Something that seemed to stand out. Something that was never seen before.

Our closest neighbour

Hubble image of Proxima Centauri with two stars nearby.
Proxima Centauri – our closest stellar neighbour. Credit: Hubble.

One of the most famous constellations in the Australian night sky is the Southern Cross. The five stars that make this small constellation appear on the Australian (and many other) flags. But on the Australian flag, there is another bright star – known as the pointer.

The Pointer is actually representative of the bright star Alpha Centauri – located not very far from the Southern Cross constellation – it’s one of the brightest stars in our night sky and visible to the naked eye all year round for southern hemisphere viewers.  

Alpha Centauri is not a single star – but rather a star system. It contains 3 stars, two of which are similar to our Sun (in terms of mass, temperature, age, and radius). The first, a yellowish-star, called Alpha Centauri and the second, a blueish-star, called Hadar. Backyard observers with a sufficiently steady telescope can resolve the pair which orbit each other approximately every 80 years.

The third star has fascinated our imagination since its discovery in 1915. It’s not bright enough to be viewed with the naked eye, and its distance from the centre of the main pair is so far – that it takes 550,000 years to complete a single orbit around the two main stars. This small, low-mass, red-dwarf star is interesting because it is currently the closest star to our solar system and an astronomically tiny 4.244 light-years from us. So close, that we can measure its physical diameter with our telescopes.

To make matters even more interesting, in 2016 the European Southern Observatory (ESO) announced that this tiny, red-dwarf, astronomically local star has an Earth-mass planet orbiting it every 11 days – and it’s at just the right distance to the parent star that it could support an environment where water could exist as a liquid (a must-have for life to form).

Flaring Up

Proxima Centauri is classified as a red dwarf star and relative to our Sun, its mass measured to be roughly 12 percent whilst its radius is only about 15%. Due to its smaller size, the fusion in its core is smaller – and this results in the star’s temperature being lower (approx.. 3,000K) giving it a reddish hue. The energy production inside Proxima Centauri is so low, it is expected to continue for an epoch of four trillion years (that is, a period greater than the existence of the entire history of the universe, as of today).

Like most dim red dwarfs, Proxima Centauri is also a flare star. This means that it can (unpredictably) undergo a rapid brightness increase across the electromagnetic spectrum (from x-rays through to radio waves) in an event that lasts a few minutes. It is believed that these flares are caused by the same process that causes our own Sun to produce solar flares – magnetic anomalies and energy within the star and its atmosphere.

Given that Proxima Centauri has an Earth-like exoplanet orbiting it, within a distance from the star where its neither too hot or too cold (termed the ‘Goldilocks Zone’), Andrew was studying the cause of these flare events (in the radio spectrum) which of course would have implications for any life existing on this exoplanet – subjected to these violent conditions.

ASKAP

Four of the ASKAP dishes, pointing upwards under an arching Milky Way.
ASKAP radio telescope. Credit: CSIRO.

The red earth of Mid-West Australia has been home to the Wajarri Yamaji people for thousands of years. The horizon stretches as far as the eye can see, with low-rising green shrubs breaking into the red hues, as a big sky hugs down on the land.

This is the location of the CSIRO’s Murchison Radio Astronomy Observatory. In particular, a powerful new instrument called the Australian Square Kilometre Array Pathfinder (or ASKAP) studies astronomical objects, events, and phenomena from this designated 520km diameter radio quiet zone – shielded from the interference of modern-day life and technology (like our mobile phone signals).

ASKAP is made of 36 radio dishes, each with a diameter of 12m. It’s frequency range – or the band that it listens in is between 700 MHz and 1.8 GHz. Each of the 36 dishes has a slightly overlapping 2-square degrees beam, giving ASKAP the opportunity to use its unique technology called the Phased Array Feed (PAF) to combine all data from each antenna and use it as a single unified instrument with a total collecting area of 4,000 square meters covering a 30-square degree field-of-view in the sky.

ASKAP’s radio view of the universe is unlike our own eyes – it captures details of cold clouds of gas that is collapsing and forming new stars, or high-energy jets zooming out of the cores of supermassive black holes in far distant galaxies. Whilst it was not utilised during Andrew's or Emil's observations, ASKAP also doesn’t need to point all its dishes in one area to observe – it can simulate the compound eyes of an agile insect, by looking in many different directions and still be recording data. This gives ASKAP a powerful advantage of covering large portions of the sky on its observing runs.

Scientists like Andrew and Emil, don’t need to be out at the Murchison observatory to control ASKAP – in fact, ASKAP can be controlled from over 3,000 km away in Sydney.

"There's something out there, pulsating"

As Emil worked away on the data being received from Andrew’s observations, he was testing a new feature on ASKAP – the ability for the instrument to see in circular polarisation. Polarisation occurs when an electromagnetic wave oscillates in one plane, and circular polarisation is where the wave rotates in a circular motion from its source. The sky itself (in general) does not produce circular polarisation light, but some objects – like flare stars and some (not all) pulsars do.

“Our eyes can’t distinguish between circularly polarised light and unpolarised light. But ASKAP has the equivalent of polaroid sunglasses that filter out the glare of thousands of garden-variety sources, and help pinpoint those rare circularly polarised jewels,” Emil said.

“It worked a treat. Proxima Centauri almost jumped out of the screen at us. And then I noticed another weaker source at the edge of the image. I had one of those ‘hmm, that’s weird’ moments.”

In an exciting rush, Emil let other colleagues from the Variable and Slow Transients (VAST) team know about his weird finding at the edge of the image, which triggered both local and international astronomers to check any available archival data for anything that might have shown signs of this particular object in this region of the sky. At the point in time when this was happening – no one in history had documented any astronomical object at this location.

Unpolarised view of the sky, with cross hairs showing the location of the Pulsar.
Unpolarised view of the sky, with crosshairs showing the location of the Pulsar. Credit: CSIRO

Polarised view of the same region, showing the variance in observation
Polarised view of the same region, showing the variance in observation. Credit: CSIRO

It took another Australian telescope, a cultural icon, on the other side of our vast continent to confirm that indeed there was an object there and that this object was a pulsar.

“Ultimately, my colleague Shi Dai, used the Parkes radio telescope to confirm that our mystery source had periodic pulses. It was indeed a newly discovered pulsar,” said Emil.

What's a pulsar?

Pulsar star illustration with two beams coming out of poles and magnetic field lines showing.
Artist illustration of a pulsar, showing beams of energy emanating from magnetic poles. Credit: Swinburne Astronomy Productions/CAASTRO.

Pulsars are both curious and extreme objects, compressed into one. Compression is the best way to describe the birth of a pulsar – the result of the imploding core, during the final moments of a massive star’s life.

When a massive star explodes as a supernova, a lot of the star is thrown off into space in a brilliantly bright detonation that can sometimes outshine entire host galaxies. However, the core of that star goes in the opposite direction – and implodes.

During this implosion, electrons (negative charges) and protons (positive charges) are forced together to form neutrons (no charge). The core of the dead star becomes a neutron star, being extremely dense and massive.

Most neutron stars have the equivalent of approx. 1.4 times the mass of the Sun squeezed into a sphere about 20km across. And here’s where things start to get very strange.

As the core collapses, it preserves the original rotational velocity of a star – but now over a much smaller area. To compensate, it spins up. Very rapidly. Think of this analogy as an example: when watching a figure skater spin on the ice with their hands extended, their velocity is impressively fast. What happens when the figure skater draws their arms in? They start to spin at an extremely fast velocity. This is exactly the same scenario.

Neutron stars and pulsars rotate on their axis hundreds of times per second. The fastest has been clocked at an astonishing 716 revolutions per second – which means its equator is moving at 24% the speed of light (that’s 72,000 km per second!)

Surely any object moving at these velocities would rip itself apart instantly? Normally, yes – this would happen. But because of the density of neutron stars and their extreme mass – they also contain extreme gravity. This assists in holding the star together. At the moment, we also don’t fully understand what the inside of neutron stars are made off (akin to Earth’s mantle) – so this must also assist in holding the star together at these extreme velocities.

In addition to the velocity being preserved during core collapse, the magnetic field of the original star also is preserved in the core – and becomes very strong. In some cases, billions of times stronger than the magnetic field of the Earth. The magnetic field of a neutron star, also like the Earth, has a north and south pole.

It is at these poles that enormous amounts of electromagnetic radiation is beamed and blasted away from the star. As the star rotates (rapidly) these beams sometimes sweep past the Earth and we see a ‘pulse’ of energy with every sweep. That’s what we call a pulsar – a star that is pulsating at us with its electromagnetic beams.

A good way to imagine this is to think of a lighthouse, set high atop a cliff. The light of the lighthouse beams away and out into across the ocean, as the lighthouse mirror rotates. When a beam passes by a ship, the ship ‘sees’ a pulse of light from the lighthouse. In our astronomical scenario, the Earth is the ship and the lighthouse is the pulsar – and the beams are the pulses.

Pulsars were discovered in 1967 by Dame Jocelyn Bell-Burnell.

Let's call it ... PSR J1431-6328!

The Pulsar that Andrew observed and Emil noticed has been designated the technical name PSR J1431-6328. The findings on this data indicate:

  • First pulsar discovered by ASKAP
  • This pulsar is approx. 1.4 times the mass of our Sun
  • It has a spin period of 2.77 milliseconds which equates to about 360 times per second (making it in the top 3% - 4% of all known pulsars)
  • It is believed this pulsar has a companion white dwarf, about 0.31 times the mass of our Sun and they orbit each other every 64 days

“There are also hints the pulsar we discovered has a nearby neighbour. Together they form a binary system that could help us test our understanding of gravity,” says Emil.

What happens now?

Parkes radio telescope dish facing skyward against a purple sunset sky
The Parkes Radio Telescope. Credit: VisitNSW.com

To build on this exciting discovery, the CSIRO team will be using the 64m Parkes radio telescope to continue following up with observations to learn more about this pulsar, and its companion.

Due to the extreme gravity associated with pulsars and pulsar systems, they provide excellent scenarios (at safe enough distances) to allow scientists to test Einstein’s General Theory of Relativity.

Utilising the circular polarisation method of observing the radio sky will also be developed further, as it now provides new opportunities to find undiscovered pulsars. There’s also the possibility of finding the things that we don’t know about as yet through this new method.

Emil says keeping an open mind to those “hmm, that’s weird” moments is critical for making new discoveries!

How far we've come, for how small we are

The next time you look up at the very bright pointer stars not far from the Southern Cross, or when you look at the Australian flag – remind yourself that it represents the closest star system to Earth, out there in the deep, lonely vastness of the cosmos.

And know that by accidental chance – we now understand that in the same direction is a tiny, yet powerful star emitting pulsations of energy that sweep past our planet and will do so for many more years to come.

That’s about the time your brain computes the perspective of our infinitesimally small presence in this infinitely grand universe.

The Paper, titled ‘Serendipitous Discovery of PSR J1431−6328 as a Highly-Polarized Point Source with the Australian SKA Pathfinder’ is currently available on arXiv