21 mins read 02 Feb 2021

Parkes Radio Telescope Spots Bizarre Magnetar in the Milky Way

Magnetars are some of the most extreme objects in the Universe. They’re tiny, dense compact stars about the size of a city, and boast magnetic fields quadrillion times more powerful than Earth’s. Now, Australian astronomers have stumbled across a curious Magnetar which is not like anything seen before.

Artist’s impression of the active magnetar Swift J1818.0-1607. Image by Carl Knox, OzGrav.

There’s no record from the humans that lived 5,000 years ago about a new or bright star emerging in the southern sky circa 3,000 BC. Though today, an ancient echo of this particular cataclysmic event scars the heavens, as an expanding and twisted shell of ejected material, which tells us that something big did indeed occur.

Its location, nestled in a neighbouring and relatively close galaxy - the Large Magellanic Cloud (LMC), is some 160,000 light-years away. 

Due to the finite speed at which light can travel, when we (or the ancient emerging Egyptian, Mesopotamian, and near east civilisations of the Early Bronze Age) look out at its light, we are seeing an image that left the LMC when Homo Sapiens only just started to wear clothing in the Middle Palaeolithic.

Yet, a supernova – a violent explosion signalling the end of the life of a massive star – took place here.

And like other supernova remnants, LMC N49 (which measures roughly 75 light-years in diameter), continued to expand into the night as scientists here on Earth studied it in detail, from about the mid-1960s using radio telescopes.

Three layer composite image of the N49 supernova remnant. In tile one, yellow and purple represent optical wavelengths. In tile two, the blue represents x-rays. The point source is thought to be the neutron star. The last tile represents a composite of both images. Credit: X-ray: NASA/CXC/Penn State/S.Park et al.). Optical: NASA/STScI/UIUC/Y.H.Chu & R Williams et al.

Then, something peculiar happened in 1979. Something so big, so powerful, that its detection was not limited to Earth, registering across the Solar system.

In early March of that year, two Soviet Venera spacecraft cruising on elliptical orbits within the inner Solar system (they had just dropped off probes at Venus) were hit with a surge of high energy pulses that only lasted a millisecond at 10:51 am EST (United States). In 0.2 seconds, an event in the distant Universe unleashed as much energy as the Sun would radiate in 10,000 years of its life.

Eleven seconds after the two Venera crafts were hit, NASA’s Helios 2 space probe, at the time orbiting around the Sun, was inundated in gamma radiation. Then, the blast traversed the planet Venus, with the Pioneer Venus Orbiter’s detectors registering the surge.

Earth was next in line and was not spared. Three Vela Satellites (operated by the US Dept. of Defense), the Soviet Prognoz 7 and the Einstein Observatory all registered the oncoming wave of high energy radiation and just before it left the Solar system, the wave passed over the International Sun-Earth Explorer.

This was the first time an enormous swell of energy was detected across the Solar system, travelling as a wave and causing instrumentation across 10 spacecraft to surge. It was at the time the strongest extra-solar gamma radiation measurement to have ever been recorded, over 100 times more powerful than anything previously registered.

Because the wave passed over several spacecraft’s instruments (that were timestamped), scientists could do some clever detective work and figure out exactly where this wave came from. Soon it became apparent, the source of the high-energy wave originated in the supernova remnant N49, in the LMC.

Coming in at the tail end of the original burst, a series of lower energy gamma rays and x-rays arrived over the course of three minutes, presenting a surprisingly regular periodicity of eight seconds. Half a day later, another small burst of high energy radiation arrived. And then, a further 16 would be detected over the course of the next four years.

We now know what caused this extraordinary event. A rare and fascinating stellar object, known as a Magnetar (this particular one is called a soft gamma repeater - SGR 0525-66). These objects are the extremely magnetised compact remains of a massive star after it has detonated in a formidable supernova explosion. The same explosion that caused LMC N49 to occur. Since then, only a handful of these objects have been discovered.

On 12 March 2020, almost exactly 41 years after the SGR 0525-66 burst, NASA’s Neil Gehrels Swift Observatory spotted a new compact object after it unleashed a massive burst of x-rays. Follow up studies revealed something extraordinary about this new object – a magnetar, called Swift J1818.0-1607 (J1818 for short) – namely that it was noticeably young (in an astronomical sense), roughly about a thousand years old.

Rapid follow-up observations also detected radio pulses originating from the magnetar and by studying the pulses, they were able to declare this a ‘radio-loud’ magnetar. Other radio-loud magnetars changed over time, scientists were able to study them to obtain a unique window into their evolution and geometry. Curiously though, astrophysicists noted that the appearance of the radio pulses from J1818 were quite different to those seen from other radio-loud magnetars.

Now, astronomers from the ARC Centre of Excellent for Gravitational Wave Discovery (OzGrav) and Australia’s national science agency, CSIRO, have reported on newly observed activity from the J1818 magnetar in a new paper outlining bizarre and never-before-seen behaviour.

“My first thought was: ‘I wonder if it emits radio pulses’, so I applied for time on Parkes to search for them. Unfortunately, our European-based colleagues beat us to that discovery.” said study lead author and Swinburne University/CSIRO Ph.D. student Marcus Lower, who specialises in studying magnetars.

The new findings suggest that magnetars actually have much more complex magnetic fields than once originally thought, which opens up the opportunity to explore and challenge current formation and evolutionary models for these extreme stellar remnants.

“Most simulations assume magnetars have relatively simple magnetic field geometries. So if real magnetars have much more complex magnetic field geometries, then our predictions for how they might evolve over time could be wrong.”

In order to better understand how J1818 would evolve over time, a team led by scientists from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) observed it eight times using the CSIRO Parkes radio telescope (also known as Murriyang) between May and October 2020.

Science Check: How Stars Evolve Into Compact Objects

The different evolutionary pathways that stars follow depending on their mass. Credit: R. N. Bailey/Wikipedia.

To grasp the background on this exciting discovery, let’s take a step back and delve into the processes around how stars evolve, die, and give birth to a new range of fascinating objects.   

The light that we see coming from the stars in the sky is a result of the thermonuclear processes occurring deep within their cores. Here, where temperatures climb into the millions of degrees, enormous pressures squeeze hydrogen nuclei together, to convert it into a new helium nucleus – thus releasing vast amounts of energy through the process known as fusion.

The ongoing conversion of hydrogen into helium provides enough energy and force radiating outwards that the star is able to balance itself against the crushing weight of gravity, trying to squeeze everything into the centre. In this stage of its life, astronomers say the star is a main-sequence star.

Eventually, the star’s hydrogen fuel runs out and things start to change for the star. What happens next is dependent on how much mass the star has to begin with.

Artist rendering of the double Red Dwarf system DG CVn. These stars are known to unleash violent flares. Credit: NASA Goddard Space Flight Centre/S. Wiessinger.

For small stars, much smaller than the Sun (known as Red Dwarfs) this process takes billions of years because the conversion through fusion takes a bit of a slower burn rate. So slow, that there are no red dwarfs which have left the main sequence stage as yet – because not enough time has passed since the Universe was created.

For stars like our Sun, the fusion rate is faster, so they don’t last as long on the main sequence stage, living for about 10 billion years (our Sun is currently halfway through its life). Now here’s where things start to get interesting.

When stars like the Sun run out of hydrogen, they start to fuse heavier elements which causes them to expand and puff. Our very own star will go through this fate, and eventually puff out enough to consume Mercury, Venus and maybe even the Earth.

The Helix Nebula - a planetary nebula 3 light-years in diameter. The remnant white dwarf can be seen at its centre. Credit: NASA, NOAO, ESA, Hubble Helix Team, M. Meixner (STScI) and T.A. Rector (NRAO).

Eventually, the Sun will puff off all of its outer layers forming a beautiful planetary nebula and leave a compact, remnant object – the hot, very dense cindered core, which is about the size of the Earth. This object is known as a White Dwarf (though, they are not always white).

But if the progenitor star is bigger than the Sun, usually about one to eight solar masses, then it consumes its hydrogen fuel relatively quickly (astronomically speaking, this is still a couple of million years). When these massive stars reach their main sequence turn off point, they also start to fuse heavier elements like helium, carbon, oxygen, magnesium, and silicon. In the process, new elements are created through this process known as stellar nucleosynthesis. 

That’s until they get to iron. Unlike all the elements that came before it, iron doesn’t give off energy – but instead absorbs energy, and so it becomes a big drain on the energy output of the star itself – the same output that keeps the star stable against that crushing force of gravity.

Suddenly, the outer parts of the star violently collapse in on the core in a cataclysmic explosion known as a supernova. The tremendous energy released from these events can be seen at phenomenal distances, and from a time when the Universe was much younger.

Artist rendition of a neutron star placed next to Manhattan Island. These objects are only 20 km in diameter, yet pack over 1.4 times the mass of our Sun inside them. Credit: NASA Goddard Space Flight Centre.

What’s even more fascinating, is what is left behind. The forces of the collapse are so great that everything gets squeezed together – including protons and electrons – to form neutrons, which are heavier in mass. The outcome is an extremely high mass and dense compact object (they’re only about the size of a city – 20 km across), known as a Neutron Star.

So dense, that if you could take a teaspoon of this material, just a single, small teaspoon – it would weigh as much as Mount Everest. And because of their high mass and density, neutron stars also have immense gravity – about two billion times stronger than the force you are experiencing right now, due to the Earth.

Neutron stars also spin fast – with some rotating about their axis hundreds of times per second. This is a result of the infalling material occupying a smaller space after the collapse, which can be described by a process known as the conservation of angular momentum. 

Most of us have likely seen this in action on TV – when we watch a figure skater spinning on the ice with their arms extended. When the skater draws their arms in, their spin velocity increases dramatically – it’s the exact same process, except the material from the outer parts of the massive star is falling inwards into the core.

The other thing that neutron stars have are powerful magnetic fields. Much like the spin velocity which is conserved during the collapse of the massive star, the magnetic field which once occupied a larger volume also compresses down and intensifies during the collapsing stage.

Different Types of Neutron Stars

Some of the different types of neutron stars that can exist (note, there are other varieties and sub-varieties as well). Credit: NASA/JPL-Caltech.

Neutron stars themselves come in a few varieties, but the three main categories are where we first start to meet these mind-boggling objects known as magnetars.

A regular neutron star is the dense, compact remains of the supernova, which has a powerful magnetic field. They usually contain about 1.2 - 2.1 times the mass of our Sun, all squeezed into a small radius the size of a city. The only thing denser that exists in the Universe are black holes, which are produced by the same supernova/collapse process but from even larger progenitor stars.

Neutron stars also have extremely powerful magnetic fields. Some of the strongest magnetic fields generated on Earth by humans have reached a few hundred thousand gauss (the unit of measurement of a magnetic field). Neutron stars, however, have magnetic fields that come in at a trillion gauss. For comparison, the MRI machine you need to visit when at a hospital, is a tiny 10,000 gauss.

But regular neutron stars can get even more mind-boggling. If a neutron star is spinning fast enough, it can generate an even more powerful magnetic field, which in turn can accelerate particles to such high velocities that they start to emit electromagnetic radiation. In particular, in the radio frequencies (but also in high-energy frequencies too).

These radio emissions are blasted out into space from the magnetic poles of the rapidly rotating neutron star, and when these beams of energy sweep past the Earth, we record them as a radio pulse. It’s a bit like a lighthouse, who has a beam which sweeps past a ship way out at sea. For this reason, these neutron stars are called Pulsars.

Radio astronomers on Earth can track pulses from pulsars with incredible accuracy and use these signals to time the regular intervals between each pulse. From this, a whole lot of fun science can be established – including measuring how much the pulsar is slowing down by each second and using the beam to tell us about what the environment is like between the observatory and pulsar.

Which then leads us to the final category of neutron stars and the star of this show – the Magnetar.

So, What is a Magnetar?

Artist impression of the magnetar in the star cluster Westerlund 1. Credit: ESO/L.Calçada.

After following through the etymology of naming pulsars after the pulses they emit, it naturally comes to name magnetars after their impressive and extraordinary magnetic fields. These objects, also neutron stars, rotate much slower than regular pulsars (and many times slower than a subclass of pulsars known as millisecond pulsars).

The rotation period normally falls between one to 10 seconds – which is still astonishingly fast when we are considering the amount of mass the object has (remember, neutron stars mass range is roughly 1.2 - 2.1 times the mass of a Sun, squeezed into a ball with 10km radius) and a simple calculation shows even at these rotation periods, the equator is moving at a speed range of 452,300 km/h – 2.2 million km/h.

“The incredibly powerful magnetic fields can cause the surface of a magnetar to crack, causing a gamma-ray burst. Such bursts would strip away the ozone layer of any nearby, Earth-like planets.”

One of the main (and wonderfully terrifying) features of a magnetar – its magnetic field – has been measured at roughly 1013 – 1015 gauss, with an absolute upper limit of 1017 gauss calculated.

To draw comparisons with familiar objects so our brains can perceive these enormous numbers, the Earth’s magnetic field at its surface (the stuff that makes your compass point north) is about 0.25 – 0.60 gauss. The family photo using a magnet to stick to your refrigerator right now is about 50 gauss, and an iron magnet is about 100 gauss. So, the magnetic fields of magnetars are quadrillions of times more powerful than these objects that we are used to.

These magnetic fields are so strong, that if we were to magically place one at the halfway point between the Earth and Moon (about 192,000 km away) the magnetar would strip all the information away from every credit card on Earth. Any closer and things start to get really terrifying. From 1,000 km away, the magnetar’s field would start to distort the electrons in the atoms in our bodies, dissolving life as we know it (and this doesn’t even incorporate the massive dose of radiation or the mess the gravitational field would make of our entire Solar system).

Artist rendition of the Magnetar SGR 0418+5729 with a magnetic loop. Credit: ESA/ATG medialab.

Emissions from magnetars, usually observed in x-rays and gamma rays, are the result of the magnetic field decaying. It is also because of this decay, that magnetars don’t last for very long (i.e. they evolve into older neutron stars, with less intense magnetic fields and high energy emissions). This process is thought to take place over tens of thousands of years – so all the magnetars we observe today were formed within the last 10,000 years (as a rough estimate), thus making them young neutron stars.

Magnetars are relatively new when it comes to their scientific confirmation, having only been identified over the last 30 – 40 years through a series of detections of high energy bursts that have travelled across the Solar system, and confirmation linking these bursts with neutron stars falling within this period.

They’re also rare when it comes to the sample population we know about – with only 30 or so that have been confirmed

“Most magnetars are located quite far away from us, so the radiation from them ends up being absorbed and scattered by the gas and dust that’s in the way. They’re also only active for a few months to years at a time, so you have to be watching the sky at just the right time to catch them after they’ve woken up,” said Marcus.

The closest magnetar to Earth is about 8,100 light-years away in the constellation of Sagittarius, and scientists have even detected one from the other side of the galaxy, at 50,000 light-years away in the direction of the constellation, Sagittarius. Recently, astronomers have also announced the likely detection of a magnetar in the Sculptor Galaxy, located about 11.5 million light-years away in the southern sky.

Except, it’s not like other Magnetars…

Composite image of Swift J1818.0-1607 as seen by EPIC-pn camera on ESA’s XMM Newton space observatory. Credit: ESA/XMM-Newton/P. Esposito et al. (2020).

In the more normal context, radio pulses from magnetars usually showcase brightness across a wide range of frequencies. But that is not the case for J1818, in which pulses were much brighter at lower frequencies, as opposed to higher frequencies. This gave this particular magnetar very pulsar-like behaviour.

Located in the constellation of Sagittarius, about 16,000 light-years away, J1818 underwent a brief and bizarre identity crisis during Marcus’s observation period. This included emitting the pulsar-like pulses in May of 2020, followed by flickering between a bright and weak state in June, and eventually, by July this peaked with J1818 flicking back and forth between emitting pulsar-like and magnetar-like radio pulses.

“This bizarre behaviour has never been seen before in any other radio-loud magnetar,” explains Marcus. “It appears to have only been a short-lived phenomenon as by our next observation it had settled permanently into this new magnetar-like state.”

Even more interesting, the observations that Marcus and his team conducted in 2020 with the Parkes radio telescope found that unlike most pulsars and magnetars, where the beams of energy emanate from the poles of the star, J1818 did not fall into this category.

“From our observations, we found that the magnetic axis of J1818 isn’t aligned with its rotation axis,” says Marcus.

“Instead, the radio-emitting magnetic pole appears to be in its southern hemisphere, located just below the equator. Most other magnetars have magnetic fields that are aligned with their spin axes or are a little ambiguous.”

“This is the first time we have definitively seen a magnetar with a misaligned magnetic pole.”

This latest study of J1818 suggests that instead of having the regular dipolar structure of the magnetic field (where there are a north and south pole on opposite ends of the structure), this magnetar has two closely spaced magnetic poles, like that seen on a horseshoe magnet.

This peculiar magnetic field configuration is also supported by an independent study of the X-rays pulses from J1818 that were detected by the NICER telescope onboard the International Space Station. The X-rays appear to come from either a single distorted region of magnetic field lines that emerge from the magnetar surface or two smaller, but closely spaced, regions.

The 1 August 2020 observation of J1818 particularly stands out to Marcus and the team. “Our best geometric model for this date suggests that the radio beam briefly flipped over to a completely different magnetic pole located in the northern hemisphere of the magnetar,” says Marcus.

The ability to observe this ‘flipping’ has the team excited, as it could present the first opportunity to be able to map the magnetic field of J1818, and start to evolve computer simulation of how magnetars are created and their evolution over time.

Links to Fast Radio Bursts

A potential mechanism for fast radio bursts to be created by magnetars. Credit: A. Weltman/A. Walters/Nature (2020), 587, p.43.

In April 2020, with much excitement, a magnetar was linked to a new astrophysical phenomenon known as Fast Radio Bursts (FRBs). These unique, millisecond bursts of radio waves have been detected by radio telescopes since the early 2000s, but their progenitor source had, up until this point, not been confirmed.

A number of theories were put forward like giant pulses from pulsars, a burst of energy from merging neutron stars, even alien signals were at one point considered (and debunked). However, all FRBs detected up until this point seemed to originate at great distances, in galaxies far away from the Milky Way.

Then, on 20 April 2020, orbiting and ground-based observatories detected FRB and an x-ray burst from the same source location in the sky, a catalogued magnetar known as SGR 1935+2154. Interestingly, this source was only 30,000 light-years away – meaning it originated from within the Milky Way Galaxy.

Several ground and space-based telescopes continued to observe the target, detecting further signals in x-rays and radios, and thus providing the first evidence that the mysterious FRBs do in fact originate from magnetars. A number of peer-reviewed papers have since appeared in astrophysics journals about the event as well.

“It wasn’t too surprising that this radio burst came from a magnetar. There’s a lot of energy stored within their magnetic fields. That it was found to have come from a known magnetar in the Milky-Way was a bit of a shock.”

In another famous example, another magnetar (PSR J1745-2900) was found (in 2013) to be orbiting the supermassive black hole that resides in the centre of our galaxy, known as Sgr A* (pronounced “Sagittarius A-star”) using three European large dish radio telescopes.

This peculiar magnetar orbits a mere 0.33 lightyears away from the monster in the heart of the galaxy, with a magnetic field of 1014 gauss. Scientists hope to continue to study PSR J1745-2900 to better understand how the interstellar medium behaves in the direction of the galactic centre.

The Future Science of Magnetars

Composite image of the southern night sky over the CSIRO Parkes radio telescope. Credit: CSIROscope/CSIRO.

Magnetars are continually challenging our knowledge of compact objects, and their behaviour and effects across their localised environments like supernova remnants are changing how matter and energy interplay with each other under extreme conditions – which could never be simulated or created on Earth (after all, none of us want to dissolve into a string of atoms).

With only 30 of these compact stellar remnants discovered, each new find in the future might once again change our understanding of stellar evolution models, how magnetic fields behave, and what happens to compact objects like pulsars and magnetars as they age.

“If we’re lucky, we might catch J1818 during one of these flips between magnetic poles sometime in the next year. That would enable us to create a map of the magnetic field structure between the two magnetic poles, something that’s never been done before.”

Marcus has the future discoveries of magnetars and behaviour of compact objects set in his sights, with his future research – wanting to delve deeper and further into the so many unknown gaps in knowledge that orbit around these magnetic monsters.  

“It’s hard to predict exactly what the next thing we’ll discover about these objects. Magnetars are a bit like cats, you can never be too sure what they’ll do next.”

It’s fairly amazing that since that powerful wave of radiation swept through the Solar system in 1979, astronomers have been able to find a whole new type of star which exhibits the most powerful magnetic field in the Universe and start to tie in a range of phenomena – like fast radio bursts – with these bizarre, yet equally fascinating objects.

The field is still very young, and with astrophysicists like Marcus working on this line of research, who knows what secrets these stars will next reveal.

Thankfully, no magnetars are destined to hang out in our neck of the woods anytime in the near future, so we’ll not have to worry about the havoc they’d wreak upon our Solar system.

Sadly, this also means we can’t just yet get rid of all of our credit card spending balances either.

Read the paper, now available on the journal Monthly Notices of the Royal Astronomical Society