Science / Astronomers discover unusual signals coming from the Milky Way's centre

Science Desk: Something in the chaotic heart of the Milky Way is flashing Earth. It beams radio waves our way for weeks, only to disappear the next day and stay dark for months. Astronomers first spotted the flakey radio emitter in 2020, and after more than a year of intermittent observations have been able to confirm little more than that the object is deeply strange. It resists categorization as one of the usual suspects, leading researchers to wonder if it might represent an entirely new type of astronomical object.

“The signal switches on and off apparently at random,” said Ziteng Wang, the University of Sydney PhD student who identified the radio source, in a press release. “We’ve never seen anything like it.”

Going where the action is

The signal popped up during a pilot survey using the Australian Square Kilometer Array Pathfinder (ASKAP), organized by Tara Murphy of the University of Sydney and David Kaplan  of the University of Wisconsin Milwaukee. Eventually, the astronomers hope to use the newly constructed array of 36 dishes, each standing three stories tall and spanning nearly 40 feet across, to scour a quarter of the Australian sky for radio sources that change monthly. But as ASKAP ramps up, they decided to focus their pilot on the galactic center, where gravity draws all manner of exotic objects to a sort of cosmic watering hole.

“We know there’s a lot of weird stuff that goes on near there: a supermassive black hole, magnetars, pulsars, all sorts of stuff,” Kaplan says. “We wanted to probe that.”

Wang noticed the anomaly while analyzing the survey’s first set of observations. Where ASKAP had seen nothing in October 2019, one point shone brightly in radio waves in January 2020.

In April, the researchers homed in on the spot with the single-dish Parkes Observatory, which specializes in hunting pulsars, the small, spinning stars that pulse with radio waves. But the instrument saw nothing.

They jumped to South Africa’s MeerKAT array, which can spot both persistent and pulsating signals. They checked the spot of sky periodically for months, but it stayed dark. The team had just started to lose hope when suddenly MeerKAT saw the bright point in February of this year. The signal was real.

 “Then, we hit it with everything,” Kaplan says.

Over the next few months, the astronomers continued to sporadically observe the object with MeerKAT and managed to secure time on two X-ray satellites and a near-infrared telescope. But the mystery only deepened.

A source unlike any other

The object beams out radio waves in patterns that, collectively, astronomers have never seen before. 

After shining for a few weeks early this year, it abruptly disappeared again. “The fact that it turned off so quickly, one day there and then the next day off,” Kaplan says, “tells us that it’s something even weirder going on.”

On top of unpredictably winking on and off, it sends out an unusually ordered signal. Most sources emit radio waves in random orientations, but the waves from this object come in certain orientations more often than others, a phenomenon called polarization. Polarized waves usually come from a powerful magnetic field, which can synchronize the spiraling of electrons and produce a tidy radio emission.

The typical cosmic magnets—such as red dwarf stars, magnetars, or brown dwarfs—shine brightly with other forms of radiation. In this case, however, the astronomers saw only radio waves. Searches with X-ray satellites and an infrared instrument turned up no sign of the mystery object, hinting that it’s some novel variety of magnetic system.

A new weirdo

Based on the team’s observations, which they described in The Astrophysical Journal on Tuesday, Kaplan feels confident in a few conclusions about the object.

It must be small, or it wouldn’t be able to disappear in a single day. And its polarization pattern suggests that a mighty magnetic field envelops the source.

The object almost certainly sits in the Milky Way (and not in some background galaxy), he says. Interstellar dust messes with polarized light as it travels through space, and the polarized radio waves from this source show little sign of interference, so they probably haven’t traveled far.  

It’s also highly unlikely to be an extraterrestrial beacon. When we communicate, we use radio waves of a single frequency. This signal contains a wide range of radio waves, like many natural sources.

Having ruled out the most obvious candidates, the mystery object may fall by default into what Kaplan calls a “grab bag of weirdos,” the so-called Galactic Center Radio Transients, or GCRTs. This handful of anomalies are the unsolved mysterious from previous radio surveys of the Milky Way’s center. Each emits distinctly, but they all share the same general behavior of flashing polarized radio waves in seemingly random ways.

The emerging class of GCRTs could mean that radio astronomers stand on the brink of uncovering a new type of cosmic drama that goes on mainly in the galactic core, such as a never-before-seen variety of white dwarf. Or they could be a coincidence generated by too many radio surveys overlapping in the heart of the Milky Way and neglecting to cover the galaxy’s outer reaches.

Future surveys by ASKAP and other arrays will pick up radio waves from more directions, more frequently, potentially helping astronomers work out whether GCRT’s mark a discovery or a mirage.

“We hope there are more of them out there so we can actually understand what’s going on,” Kaplan says. “The radio sky is really untapped.”