Something near the center of our galaxy, the Milky Way, is spewing out bizarre radio signals that have flummoxed scientists, reports a new study.
The unidentified radio source flashes for weeks at a time only to suddenly go dark—a pattern that does not line up any known space objects. The discovery is just the latest oddity that has been revealed by increasingly sensitive radio telescopes, which have exposed intricacies about enigmatic fast radio bursts and identified enormous formations called Odd Radio Circles.
Scientists led by Ziteng Wang, a PhD student in astronomy at the University of Sydney in Australia, first spotted the strange object in observations captured by the Australian Square Kilometre Array Pathfinder (ASKAP), a sophisticated radio array located in western Australia.
The mysterious source, known as ASKAP J173608.2−321635, “may represent part of a new class of objects being discovered through radio imaging surveys,” according to a forthcoming study in The Astrophysical Journal, which has been posted to the preprint server arXiv.
“The strangest property of this source is that it is highly polarized,” Wang said in an email. “Our eye cannot distinguish between circularly polarized and unpolarized light, but ASKAP has the equivalent of polarized sunglasses to filter it out. These kinds of sources are really rare.”
“Adding to the mystery, the source of the radio signals turns on and off irregularly,” he continued. “The brightness of this source can change dramatically, declining in a single day, but sometimes it can last for a few weeks.”
The team witnessed the source flaring up six times in ASKAP's data from January to September 2020, with a signal strength that varied by a factor of 100. To get a better read on the puzzling object, the researchers requested observation time on a wide range of powerful telescopes currently in operation. They were able to spot it this past February with South Africa’s MeerKAT telescope and again in April with the Australia Telescope Compact Array.
Wang and his colleagues also looked for counterparts of the radio signals in other wavelengths, such as infrared or X-ray light, using two NASA space telescopes: the Neil Gehrels Swift Observatory and Chandra X-ray Observatory. Interestingly, no signs of ASKAP J173608.2−321635 showed up in these bands of the electromagnetic spectrum, which further compounds the dilemma of its source.
“As the source was discovered using radio telescopes and has not been seen in other wavelengths, further followup observations in other wavelengths would help us reveal the nature of the source,” Wang said. “For example, a higher resolution observation would help us locate the source more accurately and could potentially help us find the counterpart in optical wavelengths.”
The team considered many possible origins for the radio signals, but each explanation has its own drawbacks.
Some low-mass stars have been observed spitting out polarized radio flares, but unlike ASKAP J173608.2−321635, they flash across other wavelengths. The source’s signals are also reminiscent of those emitted by pulsars or magnetars, which are rapidly spinning dead stars. However, these objects normally produce some kind of regular rhythm, while the new source can flash for weeks, turn off in a day, and appear absent for months.
“We thought this source might be a pulsar (a dead star) or a flaring star based on what we saw in VAST data,” Wang said, referring to ASKAP’s VAST survey. “But further observations showed these two scenarios cannot explain the behavior of the source.”
Another tantalizing possibility is that ASKAP J173608.2−321635 is a type of unidentified object called a Galactic Center Radio Transient (GCRT). Scientists first spotted these short-lived radio events 20 years ago, and a handful of them have been identified near the galactic center since then.
ASKAP J173608.2−321635 and GCRTs both emit highly polarized radio light with no X-ray counterpart, but the new source also differs from known GCRTs in the patterns and timescales of its signals. It’s also not at all clear that GCRTs even share a common origin, making it difficult to match ASKAP J173608.2−321635 to these little-understood phenomena.
“We haven’t seen how the source turns on—if we are lucky enough to see that, we might be able to know more about this source,” Wang noted.
Ultimately, no known object fully explains the weird signals. Future observations might shed more light on this mystery object at the center of the Milky Way—and perhaps find more objects like it—which “can help us better understand extreme astrophysical phenomena,” according to the study.
“If this source is an example of a previously undiscovered class of object, it would be interesting to study these types of sources to further understand their origin,” Wang concluded. “We might be able to use this kind of source as a clue to research something exciting, such as the expansion of the universe or the fate of stars.”