Mysterious object found at centre of galaxy emits rare radio signal: ‘We’ve never seen anything like it’

The transient object in the centre of the Milky Way disappeared in a single day

Mysterious radio signals coming from a source that abruptly switches itself on and off “growing 100 times brighter and dimmer over time” in the centre of our galaxy have flummoxed astronomers.

The strange source has been monitored by scientists from the University of Sydney and the University of Wisconsin-Milwaukee since last year, who noted their findings in research published in The Astrophysical Journal on Tuesday.

No definitive answers about the nature of the source, however, have emerged from intensive research that prompted the scientists to go telescope-hopping across the world.

“In early 2020, we detected an unusual radio signal coming from somewhere near the centre of our galaxy,” wrote Ziteng Wang and Tara Murphy from the University of Sydney and David Kaplan from the University of Wisconsin-Milwaukee in a piece for The Conversation.

“The signal blinked on and off, growing 100 times brighter and dimmer over time,” they wrote.

The scientists also said the patterns of the radio waves did not fit into any current classifications of radio sources from space.

“We’ve never seen anything like it,” said Mr Wang, a PhD student and the study’s lead author, in a press release.

The scientists said the unknown nature of the source and the radio waves meant this strange object could only be “a new class of stellar object.”

Objects in outer space are usually stable in terms of human time scales, the scientists explained.

So, when objects that change, called variables, and ones that appear or disappear, called transients, are spotted, they become a source of interest for astronomers.

The mysterious radio signal emitted by the source had “very high polarisation,” something exceedingly rare, according to the scientists.

High polarisation meant the signal’s light oscillated in only one direction, but the direction itself rotated with time.

The scientists moved their observations from the Australian Square Kilometre Array Pathfinder (ASKAP) telescope in western Australia to other telescopes across the world.

An international team, including scientists from Australia’s national science agency CSIRO, Germany, the US, Canada, South Africa, Spain and France, soon became involved.

While the source of the signal was discovered from the ASKAP telescope, the scientists trained one of the more sensitive ones, the MeerKAT, located at the South African Radio Astronomy Observatory, towards the object and found it disappeared in a single day.

The object has been named ASKAP J173608.2-321635, after the coordinates that the scientists were able to locate.

“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,” said professor Murphy.

The scientists had detected six radio signals from the source in a span of nine months, after which the object was located from the MeerKAT.

“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,” she said.

The scientists have moved away from some possibilities of the nature of the object, including it being a pulsar (remnant of an exploded star) or a red dwarf star.

Both these objects have polarised radio sources, but none match the mysterious nature of the object close to the centre of the galaxy.

“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,” Mr Wang said.

The scientists will now bank upon the transcontinental Square Kilometre Array radio telescope, that will “make sensitive maps of the sky everyday” to find out what the mysterious blinking object really is, according to Mr Wang.

“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,” he added.