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Astronomers have discovered unusual signals coming from the direction of the Milky Way鈥檚 centre. The radio waves fit no currently understood pattern of variable radio source and could suggest a new class of stellar object.

鈥淭he strangest property of this new signal is that it is has a very high polarisation. This means its light oscillates in only one direction, but that direction rotates with time,鈥� said Ziteng Wang, lead author of the new study and a PhD student in the School of Physics at the University of Sydney.

鈥淭he brightness of the object also varies dramatically, by a factor of 100, and the signal switches on and off apparently at random. We鈥檝e never seen anything like it.鈥�

Many types of star emit variable light across the electromagnetic spectrum. With tremendous advances in radio astronomy, the study of variable or transient objects in radio waves is a huge field of study helping us to reveal the secrets of the Universe. Pulsars, supernovae, flaring stars and fast radio bursts are all types of astronomical objects whose brightness varies.

鈥淎t first we thought it could be a pulsar 鈥� a very dense type of spinning dead star 鈥� or else a type of star that emits huge solar flares. But the signals from this new source don鈥檛 match what we expect from these types of celestial objects,鈥� Mr Wang said.

The discovery of the object has been published today in the聽.

Mr Wang and an international team, including scientists from Australia鈥檚 national science agency CSIRO, Germany, the United States, Canada, South Africa, Spain and France discovered the object using the聽聽in Western Australia. Follow-up observations were with the South African Radio Astronomy Observatory鈥檚聽听迟别濒别蝉肠辞辫别.

Mr Wang鈥檚 PhD supervisor is聽Professor Tara Murphy聽also from the聽聽and the School of Physics.

Professor Murphy said: 鈥淲e have been surveying the sky with ASKAP to find unusual new objects with a project known as Variables and Slow Transients (), throughout 2020 and 2021.聽

鈥淟ooking 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.鈥�

After detecting six radio signals from the source over nine months in 2020, the astronomers tried to find the object in visual light. They found nothing.

They turned to the Parkes radio telescope and again failed to detect the source.

Professor Murphy said: 鈥淲e 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.

鈥淟uckily, the signal returned, but we found that the behaviour of the source was dramatically different 鈥� the source disappeared in a single day, even though it had lasted for weeks in our previous ASKAP observations.鈥�

However, this further discovery did not reveal much more about the secrets of this transient radio source.

鈥淭he information we do have has some parallels with another emerging class of mysterious objects known as Galactic Centre Radio Transients, including one dubbed the 鈥榗osmic burper鈥�,鈥� said Mr Wang鈥檚 co-supervisor,聽聽from the University of Wisconsin-Milwaukee.

鈥淲hile our new object, ASKAP J173608.2-321635, does share some properties with GCRTs there are also differences. And we don鈥檛 really understand those sources, anyway, so this adds to the mystery.鈥�

The scientists plan to keep a close eye on the object to look for more clues as to what it might be.

鈥淲ithin the next decade, the transcontinental聽聽(SKA) radio telescope will come online. It will be able to make sensitive maps of the sky every day,鈥� Professor Murphy said. 鈥淲e 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.鈥�

Declaration

Researchers received funding and support from the following bodies: Australian Research Council, US National Science Foundation, European Research Council, Natural Sciences and Engineering Research Council of Canada and the Sydney Informatics Hub at the University of Sydney.

The ASKAP radio telescope is part of the Australia Telescope National Facility which is managed by CSIRO. Operation of ASKAP is funded by the Australian Government with support from the National Collaborative Research Infrastructure Strategy. The MeerKAT telescope is operated by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation.

The researchers gratefully acknowledge the Wajarri Yamatji as traditional owners of the Murchison Radio-astronomy Observatory site, on which ASKAP is located.