Mysterious signals unlike anything ever seen before have been detected coming from the direction of the galactic centre, baffling astronomers.
The signal "switches on and off apparently at random" and the object it's coming from is invisible, according to a new paper published this week.
"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," said Ziteng Wang, University of Sydney PhD student and lead author.
"But the signals from this new source don't match what we expect from these types of celestial objects."
Given the catchy name ASKAP J173608.2-321635, named after its coordinates in the sky, the object was first spotted in 2021 using the Australian Commonwealth Scientific and Industrial Research Organisation's ASKAP radio telescope in Western Australia.
"Looking towards the centre of the galaxy, we found ASKAP J173608.2-321635," said Wang's PhD supervisor Tara Murphy. "This object was unique in that it started out invisible, became bright, faded away and then reappeared. This behaviour was extraordinary."
When Prof Murphy says "became bright", she means in the radio spectrum - increasing its output by a factor of 100, before disappearing. It doesn't appear to give off any visual or infrared energy, making it invisible to the naked eye. It also couldn't be detected using telescopes designed to find X-ray signals, Wang said.
The strange signal was picked up six more times in 2020, then went quiet. When it returned, the mystery only deepened.
"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," said Prof Murphy.
The paper, published in The Astrophysics Journal on Tuesday, suggests it might be "a low-mass star/substellar object with extremely low infrared luminosity, a pulsar with scatter-broadened pulses, a transient magnetar, or a Galactic Center radio transient [GCRT]". But none of those explanations really fit, according to what astronomers currently know about them.
The latter seems the best candidate, with one of the team suggesting the new signal has "some parallels" with a previously discovered signal dubbed the 'Cosmic Burper', including its location.
"While our new object... does share some properties with GCRTs there are also differences," said Wang's co-supervisor, David Kaplan from the University of Wisconsin-Milwaukee. "And we don't really understand those sources, anyway, so this adds to the mystery."
More clues might be found when the Square Kilometre Array telescope - which will have dishes across the globe - goes online sometime in the next decade.