- Mysterious radio waves emanating from the center of the galaxy have astronomers stumped.
- Four objects have briefly emitted radio signals that don't resemble any known type of star.
- Scientists think each of the four signals could come from a new type of object unknown to astronomy.
Ziteng Wang found a needle in an astronomical haystack.
Wang, a physics PhD student at the University of Sydney, was combing through data from Australia's ASKAP radio telescope in late 2020. His research team had detected 2 million objects with the telescope and was classifying each one.
The computer identified most of the stars, and the stage of life or death they were in. It picked out telltale signs of a pulsar (a rapidly rotating dead star), for example, or a supernova explosion. But one object in the center of our galaxy stumped the computer and the researchers.
The object emitted powerful radio waves throughout 2020 — six signals over nine months. Its irregular pattern and polarized radio emissions didn't look like anything the researchers had seen before.
Even stranger, they couldn't find the object in X-ray, visible, or infrared light. They lost the radio signal, too, despite listening for months with two different radio telescopes.
It reappeared suddenly, about a year after they first detected it, but within a day, it was gone again.
"Unfortunately, we don't quite know what behaves like that," Tara Murphy, a professor at the University of Sydney who led Wang's research team, told Insider.
It was becoming clear that this was no ordinary dead star, like the other 2 million objects in their survey.
"That's when we started getting excited," Murphy said.
The team sent their data to other radio astronomers, asking for theories. Bit by bit, they confirmed that no one had ever detected anything like it before.
The researchers' conclusion: The discovery could belong to an obscure category of mysterious signals coming from the core of the Milky Way, known as "galactic center radio transients" (GCRTs). Before Wang's discovery, only three such objects had ever been identified.
The name GCRT is a "position holder," Murphy said, "while we actually try and figure out what they are."
Murphy is "100% confident" that the signals aren't coming from aliens, because technological signals would cover a much narrower range of frequencies, like humans' broadcast radios do.
GCRTs have been a mystery for decades now. Nobody knows what type of star would make those unique signals, and each GCRT is different, leading researchers to believe the four signals are not coming from the same type of object.
Any new discovery "adds to the full body of knowledge that either cements what we already know, or adds to it, or really could lead to revolutionary new understandings," Scott Hyman, who led the research efforts that discovered the three prior GCRTs, told Insider. "Whether these objects fall into those categories, we don't know. We don't know enough about them."
A decade of searching only turned up 3 GCRTs
Telescopes first began observing the center of the Milky Way in low radio frequencies in the 1990s. But it wasn't until the early 2000s, when Hyman's research team was studying data archives from such a low-frequency radio telescope, that they discovered a strange signal briefly beaming from the galactic center.
The signal got stronger and then faded away over the course of a few months. Unlike other transient radio signals, there was no sign of it in X-ray observations.
Hyman and his colleagues had discovered the first GCRT. Within three years, the team found another, which they nicknamed "the burper" for the radio bursts it sent out every 77 hours before disappearing.
These were extremely "bright" signals, meaning they emitted powerful radio waves. Hyman figured they'd find many more GCRTs if they kept searching, including "dimmer," or weaker ones.
"We thought we were at the tip of an iceberg," said Hyman, who is retired but formerly worked as a physics professor and researcher at Sweet Briar College. "We expected that, given the first one was so easy to find, that we would find more. But I think we were just lucky."
In about 10 years of searching, they only found one more GCRT. It, too, was hidden in archival data. They studied the skies anew with the Very Large Array radio telescopes, but none of their signals turned up again.
Wang and Murphy may have finally found another GCRT, but their discovery didn't shed much light on what these mysterious objects might be.
Astronomers only have 'unsatisfying' theories until they discover more GCRTs
Researchers have theories about the GCRTs, but "none of them are very satisfying," Murphy said.
The GCRTs could be neutron stars or pulsars orbiting each other in sets of two or three, so that the radio signal from one star is eclipsed at irregular intervals by the others. They could be pulsars dying — running out of energy — and emitting irregular radio gasps.
Hyman still thinks there are other, undiscovered GCRTs, some of them obscured by the thick dust that pervades the center of the Milky Way.
New observatories are monitoring the galactic center better than Hyman could in the 2000s. Whenever the US Naval Research Laboratory releases new observations of the galaxy's center, he scans them for signs of GCRTs. Murphy's team plans to continue listening to the galactic center with ASKAP, simultaneously looking for signs of their mystery objects in X-ray, visible, or infrared light.
The Square Kilometer Array, currently under construction in Australia and South Africa, would be far more capable of finding GCRTs than any prior radio observatory, Hyman said. It's set for completion in 2028.
"I'm really hopeful that we can re-detect these three objects, figure out what they are," Hyman said. "They could be lurking in a very dim, quiescent state. They could be just very faint right now, and yet still detectable with a very sensitive instrument."