Strange radio waves from the centre of the galaxy

In the West Australian desert, an array of radio telescopes has spotted strange radio signals emanating from somewhere near the heart of the Milky Way.

These signals don’t fit any known patterns produced by familiar radio sources such as pulsars, quasars or radio galaxies. Instead, astronomers think they could come from a new type of stellar object.

According to the lead author of the new study, Ziteng Wang from the University of Sydney, the strangest property of this new signal is its high polarisation.

“This means its light oscillates in only one direction, but that direction rotates with time,” explains Wang.

“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.”

Co-author Tara Murphy, also from the University of Sydney, says this new object is “unique in that it started out invisible, became bright, faded away and then reappeared. This behaviour was extraordinary.”

The discovery was made using the Australian Square Kilometre Array Pathfinder (ASKAP) telescope in WA. Using ASKAP, Wang and team detected six radio signals from the same source over the first nine months of 2020.

They then tried to observe the same signal in visible wavelengths, hoping to “see” the source, but they saw nothing.

Next, they tried with the Parkes radio telescope, again to no avail.

The team then turned to the more sensitive MeerKAT radio telescope in South Africa, checking for the signal for 15 minutes every few weeks.

“Luckily, the signal returned,” says Murphy. “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.”

And it didn’t give the team any further hints about the origin of the mysterious signal.

The team even tried to see the source in X-ray wavelengths, using orbiting observatories such as NASA’s Swift and Chandra telescopes, but spotted nothing.

So what could be creating this signal?

Radio emissions can come from a variety of different sources, many of which are the most energetic and powerful processes in the cosmos. The new signal is variable, which cuts out many options such as stars, normal neutron stars and X-ray binary systems. But there’s still a lot of choice – from pulsars to supernovae to flaring stars to fast radio bursts.

“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,” says Wang. “But the signals from this new source don’t match what we expect from these types of celestial objects.”

Another co-author, David Kaplan from the University of Wisconsin-Milwaukee in the US, says “the information we do have has some parallels with another emerging class of mysterious objects known as Galactic Centre Radio Transients (GCRTs), including one dubbed the ‘cosmic burper’.

“While our new object…does share some properties with GCRTs, there are also differences. And we don’t really understand those sources anyway, so this adds to the mystery.”

The fact that the signals are emerging from near the centre of the galaxy is also notable, but it’s unclear whether that’s a coincidence or if the location is related to the nature of the source.

The puzzle remains unsolved. Since no known source fits the observations, it’s possible that the signals represent a new class of celestial objects.

The next generation of radio telescopes, such as the upcoming Square Kilometre Array, will conduct comprehensive surveys that might determine whether this signal is unique, or whether there are many more – and also shed light on its origin.

The discovery is published in the Astrophysical Journal.