Astronomers have been tantalised by radio signals coming from the centre of the Milky Way. The strange radio signals do not match the known pattern of radio sources, suggesting a new class of stellar objects.

The strange radio waves have been described in a paper published in The Astrophysical Journal on October 12. Ziteng Wang, the lead author of the study, a PhD student in the School of Physics of the University of Sydney, commented on the signals saying, “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.”

“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.”

Stars can emit various types of lights across the electromagnetic spectrum. Radio astronomy has advanced significantly in recent decades, and it is now possible to study different objects in the sky, which is helping scientists reveal the secrets of the universe. Radio astronomy has become a vast field of study. Pulsars, Supernova, stars and fast radio bursts are some examples of astronomical objects that radio astronomy has helped understand. These astronomical objects have differential brightness.

“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,” Wang further commented.

The paper published resulted from a collaborative effort, including scientists from Germany, the US, Canada, South Africa, Spain, and France. The study was led by Australia’s national science agency CSIRO. Scientists discovered the radio wave emanating object using the ASKAP telescope of CSIRO, with the MeerKAT telescope of the South African Radio Astronomy Observatory carrying out the follow-up observations.

The supervisor of Wang, Prof. Tara Murphy from the Sydney Institute for Astronomy and School of Physics, said about their discovery, “We have been surveying the sky with ASKAP to find unusual new objects with a project known as Variables and Slow Transients (VAST), throughout 2020 and 2021.”

“Looking towards the centre of the Galaxy, we found ASKAP J173608.2-321635, named after its coordinates. This object was unique in that it started out invisible, became bright, faded away and then reappeared. This behaviour was extraordinary.”

The team of researchers detected six radio signals from the source within nine months in 2020. The researchers also tried to find the object in visual light, but they could not find anything.

This led the team to use the more sensitive MeerKAT telescope, hoping to see the intermittent radio signals that were observable for 15 minutes every few weeks.

“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,” Prof. Murphy said.

All the efforts did not fruitfully reveal much about the transient radio source, and it remains something of an enigma for the scientists. However, the scientists plan to keep an even closer eye on the object to decipher more clues about it.