New Delhi: Strange radio signals are coming from the direction of the Milky Way Galaxy’s centre, astronomers have discovered. Scientists have studied the patterns of variable radio sources for a long time, but the new radio waves detected do not match any of those previously known patterns. 

The astronomers believe these unusual signals could suggest a new class of stellar object.

The study explaining the discovery of the object was recently published in the Astrophysical Journal Letters. 

Using the Commonwealth Scientific and Industrial Research Organisation’s ASKAP (Australian Square Kilometre Array Pathfinder) radio telescope, Ziteng Wang, the lead author of the new study and PhD student in the School of Physics at the University of Sydney, along with an international team of scientists, detected these surprising signals emerging from deep in the heart of the Milky Way Galaxy. 

What is unusual about this signal?

Ziteng Wang said what the scientists found most strange about the new signal is its “very high polarisation”, according to a statement by the University of Sydney. 

“This means its light oscillates in only one direction, but that direction rotates with time,” he explained. In simple words, polarisation is a property of light waves that depicts the direction of their oscillations.

The study author also noted that this was a never-seen-before phenomenon. A dramatic variation in the brightness of the object was observed, and the signal switched on and off at random, he said, adding that the brightness increased or decreased by a factor of 100 during those episodes. 

In order to find out the secrets of the universe, variable or transient objects in radio waves are being studied. Variable objects in astronomy are those stellar objects which change brightness, or in other words, emit variable light across the electromagnetic spectrum. Transients are astronomical phenomena which can last for durations of fractions of a second to weeks or years. Variables are detectable at any point of time, while transients fall below the detectable limit due to their short-lived nature. 

Some examples of astronomical objects whose brightness varies are pulsars, supernovae, flaring stars, and fast radio bursts. 

Pulsars are rotating neutron stars, or a dense spinning dead star, which emit pulses of radiation at very regular intervals, which may range from milliseconds to seconds.

Supernova is the explosion of a star, while fast radio bursts are highly intense bursts of radio waves produced by unidentified sources, which last for a few milliseconds.

Flare Stars are variable stars which exhibit dramatic variations in brightness, within a few minutes. 

Ziteng Wang said that they initially speculated the unusual signals to come from a pulsar, which is a very dense type of spinning dead star. He said they also speculated the signal to come from a star that emits huge solar waves. 

But they later shrugged off these assumptions because the newly discovered signals did not match the signals that are expected to be emitted by pulsars or other types of celestial objects known. 

What are the findings?

Ziteng Wang, along with scientists from Australia’s national science agency CSIRO, and from countries like Germany, the United States, Canada, South Africa, Spain and France, discovered the unusual object using CSIRO’s ASKAP radio telescope in Western Australia. They subsequently conducted further observations using the South African Radio Astronomy Observatory’s MeerKAT telescope.

Professor Tara Murphy, from the Sydney Institute for Astronomy, and the School of Physics, said they had been surveying the sky throughout 2020 and 2021 to detect new objects using ASKAP. Variables and Slow Transients (VAST) is the name of the project. 

She added that they named the newly detected unusual object after its coordinates as ASKAP J173608.2-321635. This object, coming from the centre of the galaxy, is unique because it was invisible in the beginning, then turned bright, faded away and then reappeared, she explained. She noted that the behaviour exhibited by the object was extraordinary.

Over nine months in 2020, the researchers had detected six radio signals from the object. They tried to find the object in visible light, but it was in vain. Then, they tried to detect the source using the Parkes radio telescope, but were unsuccessful at their attempts.

Murphy explained that the signal was intermittent (occured at irregular intervals), which is why they observed it for 15 minutes every few weeks, using the more sensitive MeerKAT radio telescope in South Africa, in the hope of seeing the signal again. 

Fortunately, she added, the signal did return, but it exhibited a behaviour that was dramatically different. In previous ASKAP observations, the signal had lasted for weeks, but this time, the source disappeared in a single day, she said.

The scientists note that not much is revealed about the secrets of the universe from this discovery. 

Ziteng Wang’s co-supervisor, Professor David Kaplan from the University of Wisconsin-Milwaukee, said the new discovery had some parallels with another emerging class of mysterious objects known as Galactic Centre Radio Transients (GCRTs), one of which was dubbed the ‘cosmic burper’. 

Galactic Centre Radio Transients are not one specific object but a group of objects emitting radio waves, around the Milky Way’s centre.

Astronomers speculate that the signals could be coming from GCRTs, because the new object shares some properties with this class of mysterious objects. Kaplan said there were also certain differences between the properties of the new object and GCRTs. “And we don’t really understand those sources, anyway, so this adds to the mystery,” Kaplan was quoted as saying.

The researchers aim to observe the object closely as it may provide further clues about its identity.

Professor Murphy said astronomers will be able to take sensitive maps of the sky every day using the transcontinental Square Kilometre Array (SKA) radio telescope, which is being planned to be built in Australia and South Africa. She explained that this powerful telescope will not only help astronomers solve mysteries such as the latest tantalising discovery, but will also further exploration in the radio spectrum of the cosmos.