Mysterious Radio Signal Discovered Coming from the Centre of the Galaxy

An international team of astronomers, led by The University of Sydney’s PhD student Ziteng Wang, have discovered a mysterious radio signal coming from near the centre of the Milky Way Galaxy. The signal is completely different to any currently understood source of radio waves and has left the team, and the greater astronomy community, both excited and intrigued as to what it could be.

What makes these findings so exciting is that the radio signal first appeared out of nowhere, brightened up, faded down, disappeared, then reappeared again. Not the regular type of behaviour generally observed with most celestial objects. 

From what we know about the object - its variability changed in values greater than 100 in the timescale of one week, it had persistent emissions for a few weeks but also could decline in a period as short as one day. 

To make this discovery, Ziteng and the team (which includes astronomers from Australia’s national science agency, CSIRO, Germany, the USA, Canada, South Africa, Spain and France) needed to use CSIRO’s ASKAP - the large radio frequency interferometer located in Western Australia. 

ASKAP is made up of 36 individual dishes, each 12-metres in diameter and separated, with the largest distance between any two antennas equal to about 6-kilometres. This turns the instrument into a powerful interferometer - a simulated virtual telescope with an eye that is 6-kilometres wide, helping improve resolution in observing objects out in the cosmos.

The discovery of ASKAP J173608.2-321635 (named for its sky coordinates) was serendipitous, as the object is considered a transient, that is, it doesn’t continually shine for a long period of time like the regular stars we see. There’s a whole project (one of many) dedicated to these types of phenomena, with this particular project known as the Variables and Slow Transients (VAST) project. It uses ASKAP to search for sources that might be appearing for the first time. As is the case with ASKAP J173608.2-321635 - discovered through the VAST project. Even more intriguing, this new discovery exhibited some peculiar behaviour. 

“The strangest property of this new signal is that it has a very high polarisation. This means its light oscillates in only one direction, but that direction rotates with time,” said the lead author of the study, Mr Wang.

Polarisation occurs when electromagnetic light orients itself in a certain way as it traverses, due to the effects of magnetic fields, or when it is reflected off a surface. It comes in three main orientations - linear, circular and elliptical. 

Linear polarisation occurs when the electromagnetic wave oscillates in one direction - vertically or horizontally. If you’ve ever used your hands to create a wave like motion in a skipping rope by moving it up and down, then the wave travelling down the rope is said to be vertically linear polarised. Circular polarisation is when the electromagnetic wave travels in a circular motion, resulting in a helix-spiral-like pattern. Elliptical polarisation is a cross between the two. 

 In the case of ASKAP J173608.2-321635, the object had a high degree of circular polarisation, with a very steep radio spectrum (-2.7 to -5.5), and also exhibited a high degree of linear polarisation. It also showed that the rotation measure (which outlines how the plane of polarisation is rotating) changed significantly across the space of three days. 

And whilst the polarisation of the signal was interesting, and there are many types of celestial objects that emit variable signals, such as pulsars and supernovae, the case of ASKAP J173608.2-321635 was a little different in another factor.

“The brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. We’ve never seen anything like it,” said Mr Wang.

“At first we thought it could be a pulsar – a very dense type of spinning dead star – or else a type of star that emits huge solar flares. But the signals from this new source don’t match what we expect from these types of celestial objects.”

Observations of the object started in 2020, when over the course of nine months, six radio signal detections were made before it went quiet. Hoping to catch it again, astronomers turned to other telescopes, like the Parkes radio telescope but once again failed to detect anything further. 

A search was also conducted in the near-infrared and x-ray wavelengths but revealed no clue as to what it could be.

“We then tried the more sensitive MeerKAT radio telescope in South Africa. Because the signal was intermittent, we observed it for 15 minutes every few weeks, hoping that we would see it again,” said Mr Wang’s PhD supervisor, Professor Tara Murphy.

“Luckily, the signal returned, but we found that the behaviour of the source was dramatically different – the source disappeared in a single day, even though it had lasted for weeks in our previous ASKAP observations.”

This still did not provide clues as to what the source of the signal could be.

Ziteng and his team did run through a number of different possibilities, including the possibility of a pulsar with a short period, or in an eccentric binary system, or scattering of the radio signal in the interstellar medium. Even supernovae events were considered - but with all of these options, none really confirmed what it could be as the behaviour of ASKAP J173608.2-321635 was very different from similar phenomena.

The location of where the object was found is of particular interest as well - as the central region of our Galaxy is known to cause lots of scattering and broadening of pulsar radio signals, due to the large amount of material that lies in that direction and within the region itself. 

Mr Wang’s co-supervisor, Professor David Kaplan from the University of Wisconsin-Milwaukee said “The information we do have has some parallels with another emerging class of mysterious objects known as Galactic Centre Radio Transients, including one dubbed the ‘cosmic burper’” 

While the new mysterious signal does share some properties with GCRT’s, it also has differences.

“And we don’t really understand those sources, anyway, so this adds to the mystery.” said Professor Kaplan.

Astronomers plan to continue to study the object.

“Within the next decade, the transcontinental Square Kilometre Array radio telescope will come online. It will be able to make sensitive maps of the sky every day. We expect the power of this telescope will help us solve mysteries such as this latest discovery, but it will also open vast new swathes of the cosmos to exploration in the radio spectrum.” said Professor Murphy.

We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site on which ASKAP is located, and the Wiradjuri people as the traditional owners of the location of the Parkes radio telescope.