The MWA Telescope Catches A Mysterious New Radio Object

Many of us will have fond memories of looking through a telescope at some point and seeing beautiful astronomical objects in the night sky. It might have been the Moon, featuring all those craters and mare, or Jupiter with its four star-like moons. Or maybe the jewel of the Solar system, Saturn with its dazzling display of that wonderful ring system. In any case, the mechanics are fairly straightforward – you set up your telescope, point it in the direction of the object and look through the viewfinder.

One thing that is not familiar to most people when first looking through a telescope is how fast these objects move through the field of view. This is of course an apparent effect – the objects are traversing in their orbits at great distances to us and their minute-by-minute speed is not perceivable by our backyard telescopes. Instead, it is caused by the Earth rotating on its axis and turning under the night sky.

It's the field of view that is the key here. The telescope zones in on such a small patch of the sky that it amplifies how fast that part of the sky is rolling overhead. This is important because it limits the observer (be it amateur or professional) to only observe, document, analyse or discover what is in this small field of view during this small time of observation. In short, if something exciting happens outside the field of view, it may forever be unknown. Thankfully these days, even amateur telescopes have tracking capabilities and motors that can counter Earth’s rotation once locked on target.

But not all telescopes have small fields of view. In fact, some telescopes can see the whole sky at once. For them, they can watch the stars and galaxies, night after night, collecting massive amounts of data and looking for any changes from previous observations – which can occur in repetitive periods as small as milliseconds or seconds, can sometimes last weeks, or even be one-off events, never to be seen again. They are known as transients.

These wide field-of-view instruments play an important role in the discovery of new objects, events and phenomena that happen to be transitioning through different phases of their evolution – because astronomers need to be looking at a big patch of sky for them to have a higher probability of seeing something new or changing – otherwise they would be very lucky to see it in the smaller patches of sky. In other words, if you cast the net wide, the more likely you’ll get a catch.

Now, a team of astronomers, have announced the discovery of something unusual and unlike anything, anyone has ever seen before, by using one of these odd-looking radio telescopes that keeps an eye on the entire sky (night and day).

An object that gives out three giant bursts of energy across radio wavelengths roughly three times per hour (more or less every 18 minutes). The discovery has astronomers excited as it is highly unusual for an object to undergo repetitive brightness over such long periods.  

Astrophysicist Dr Natasha Hurley-Walker, from the Curtin University node of the International Centre for Radio Astronomy Research, led the team that made the discovery.

“This object was appearing and disappearing over a few hours during our observations,” she said.

“That was completely unexpected. It was kind of spooky for an astronomer because there’s nothing known in the sky that does that.”

What the team of astrophysicists think it could be, is one of three possibilities. The remnant core of a former massive star that has now since faded after its supernova explosion, such as a neutron star. Or maybe something a little less massive, but still a remnant, like a white dwarf. And lastly, and possibly the most exciting idea – something completely new.

So why is this object, known with its astronomical name of GLEAM-X J 162759.5-523504.3 (an easy one to remember!) fascinating? Well, often things that brighten in space can be linked to a phase transition or evolutionary stage, such as an explosion like a supernova, a merger like when two stars collide, or a rotating beam of energy, like that produced by a pulsar. But none of these objects have ever been documented to occur every 18 or so minutes. After all, an object does not set itself into self-destruction mode on a periodic timescale.

But for this particular object, it does. It not only sends out a beam of radiation that crosses the Earth’s line of sight every 18 mins, when it does it is one of the brightest radio sources in the sky, along with the beam lasting for about a minute, instead of mere seconds (like a pulsar). And before your imagination takes you any further, it’s certainly not the little green men.

Initially, the object was discovered with two significant detections, but further analysis returned a total of 71 pulses ranging between the period of January to March 2018. And by measuring the number of electrons between us and the object (through a process known as dispersion measure), the team was able to establish that it’s rather local to our neighbourhood in the galaxy – a mere hop of only 4,000 light-years away, in the direction of the constellation Norma.

To make the discovery, Curtin University Honours student Tyrone O'Doherty used a new technique he developed, and the Murchison Widefield Array (MWA) instrument, which is not your everyday type of telescope – even for radio astronomy purposes. Instead of the familiar dish antennas that we often conjure in mind (like the CSIRO Parkes radio telescope), the MWA is more like lots of metallic spiders that sit out in the red Earth sands of the Australian outback.

“It’s exciting that the source I identified last year has turned out to be such a peculiar object,” said Mr O’Doherty, who is now studying for a PhD at Curtin.

“The MWA’s wide field of view and extreme sensitivity are perfect for surveying the entire sky and detecting the unexpected.”

The MWA instrument is a low-frequency radio array, working within an operating frequency of 70 – 300 MHz. The spider-like antennas, each with 16 dipoles, are arranged in a 4 x 4 configuration and sit atop a 4m x 4m mesh ground plate. This set-up is known as a tile, and collectively there are 256 tiles (so just over 4,000 antennas all working in unison).

Tiles are separated by distances – with the furthest separation (the baseline) coming to 6-kilometres, which allows the telescope to have a higher resolution, however, the majority of the tiles are located within a 1.5-kilometre core region.

A couple of advantages the telescope has includes seeing the whole southern sky from 30-degrees north, as well as being located at the CSIRO Murchison Radio-astronomy Observatory (MRO), a government dedicated radio quiet zone, where human transmissions and interference are limited. This allows for some of the most sensitive surveys of the radio sky to be conducted from anywhere in the world.

These findings were made as part of the Galactic and Extra-galactic All-sky MWA survey, also known as GLEAM.

One of the ideas being thrown around by astronomers about what the object could be, is something known as a magnetar -  which is a type of neutron star that has an impossibly strong magnetic field, even more so than your typical neutron star or pulsar. To draw a comparison, Earth’s magnetic field at its surface is measured to be roughly 0.25 – 060 gauss. The magnet on your fridge right now is about 50 gauss, and an iron magnet is about 100 gauss.

For magnetars, their magnetic fields are in the range of 10¹³ – 10¹⁵ gauss, with upper limits calculated at 10¹⁷ gauss – roughly a quadrillion times more powerful than our everyday experiences with magnets. So powerful are the magnetic fields of these objects, that if you placed one at the halfway point between the Earth and the Moon, it would strip away all the information from every credit card on our planet. Place them a little closer (let’s say about 1,000-kilometres away, and ignoring the gravitational and radiation effects) and the electrons in our bodies would start to become distorted, shredding life as we know it.

This particular object is extremely bright, yet smaller than our Sun, and the radio emissions are very highly polarised, which tells us that there must be an absolutely strong magnetic field at play here. But unlike other magnetars, the periodicity of the beam (every 18-minutes) is not like other known magnetars.

Dr Hurley-Walker said the observations match a predicted astrophysical object called an ‘ultra-long period magnetar’.

“It’s a type of slowly spinning neutron star that has been predicted to exist theoretically,” she said.

“But nobody expected to directly detect one like this because we didn’t expect them to be so bright.

“Somehow it’s converting magnetic energy to radio waves much more effectively than anything we’ve seen before.”

ICRAR-Curtin astrophysicist and co-author Dr Gemma Anderson said that “when studying transients, you’re watching the death of a massive star or the activity of the remnants it leaves behind”.”

‘Slow transients’—like supernovae—might appear over the course of a few days and disappear after a few months.

‘Fast transients’—like a type of neutron star called a pulsar—flash on and off within milliseconds or seconds.

But Dr Anderson said finding something that turned on for a minute was really weird.

A telescope like the MWA, which has a big field of view, will collect enormous volumes of data during observations, all of which requires processing.

The established data flow and management process starts at the telescope, where the antennas collect the information, before passing it to the receiver, then through to a correlator. But the MWA is located out in the middle of the desert, and building a processing and storage facility in this harsh Australian climate would be both risky and strategically disadvantaged.

So, all of the data that is collected by the MWA (as well as other telescopes at the MRO, such as CSIRO’s ASKAP) is sent onto the Pawsey Supercomputing Centre, located in Perth, for long-term storage and access by the wider global astronomy community.

These instruments, as well as this established (and successful dataflow operation), are all considered pathfinding technologies for a much more ambitious project, which will see Australia hosting the largest radio telescope in the world by the end of the decade.

Known as the Square Kilometre Array (SKA), the behemoth instrument will be split across two nations – South Africa, which will host the dish antennas, and Australia, which will host over 130,000 Christmas tree-like antennas (which act in a similar manner to the current MWA dipoles).

“Key to finding this object, and studying its detailed properties, is the fact that we have been able to collect and store all the data the MWA produces for almost the last decade at the Pawsey Research Supercomputing Centre. Being able to look back through such a massive dataset when you find an object is pretty unique in astronomy,” said MWA Director Professor Steven Tingay.

“There are, no doubt, many more gems to be discovered by the MWA and the SKA in coming years.”

We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site on which MWA is located.

Video Credits: ICRAR