Slowly repeating bursts of intense radio waves from space have puzzled astronomers since they were discovered in 2022.

In new research, my colleagues and I have for the first time tracked one of these pulsating signals back to its source: a common kind of lightweight star called a red dwarf, likely in a binary orbit with a white dwarf, the core of another star that exploded long ago.

A Slowly Pulsing Mystery

In 2022, our team made an amazing discovery. Periodic radio pulsations that repeated every 18 minutes, emanating from space. The pulses outshone everything nearby, flashed brilliantly for three months, then disappeared.

We know some repeating radio signals come from a kind of neutron star called a radio pulsar, which spins rapidly (typically once a second or faster), beaming out radio waves like a lighthouse. The trouble is, our current theories say a pulsar spinning only once every 18 minutes should not produce radio waves.

So we thought our 2022 discovery could point to new and exciting physics—or help explain exactly how pulsars emit radiation, which despite 50 years of research is still not understood very well.

More slowly blinking radio sources have been discovered since then. There are now about 10 known “long-period radio transients.”

However, just finding more hasn’t been enough to solve the mystery.

Searching the Outskirts of the Galaxy

Until now, every one of these sources has been found deep in the heart of the Milky Way.

This makes it very hard to figure out what kind of star or object produces the radio waves, because there are thousands of stars in a small area. Any one of them could be responsible for the signal, or none of them.

So, we started a campaign to scan the skies with the Murchison Widefield Array radio telescope in Western Australia, which can observe 1,000 square degrees of the sky every minute. An undergraduate student at Curtin University, Csanád Horváth, processed data covering half of the sky, looking for these elusive signals in more sparsely populated regions of the Milky Way.

And sure enough, we found a new source! Dubbed GLEAM-X J0704-37, it produces minute-long pulses of radio waves, just like other long-period radio transients. However, these pulses repeat only once every 2.9 hours, making it the slowest long-period radio transient found so far.

Where Are the Radio Waves Coming From?

We performed follow-up observations with the MeerKAT telescope in South Africa, the most sensitive radio telescope in the southern hemisphere. These pinpointed the location of the radio waves precisely: They were coming from a red dwarf star. These stars are incredibly common, making up 70 percent of the stars in the Milky Way, but they are so faint that not a single one is visible to the naked eye.

Combining historical observations from the Murchison Widefield Array and new MeerKAT monitoring data, we found that the pulses arrive a little earlier and a little later in a repeating pattern. This probably indicates that the radio emitter isn’t the red dwarf itself, but rather an unseen object in a binary orbit with it.

Based on previous studies of the evolution of stars, we think this invisible radio emitter is most likely to be a white dwarf, which is the final endpoint of small to medium-sized stars like our own sun. If it were a neutron star or a black hole, the explosion that created it would have been so large it should have disrupted the orbit.

It Takes Two to Tango

So, how do a red dwarf and a white dwarf generate a radio signal?

The red dwarf probably produces a stellar wind of charged particles, just like our sun does. When the wind hits the white dwarf’s magnetic field, it would be accelerated, producing radio waves.

This could be similar to how the Sun’s stellar wind interacts with Earth’s magnetic field to produce beautiful aurora and also low-frequency radio waves.

We already know of a few systems like this, such as AR Scorpii, where variations in the brightness of the red dwarf imply that the companion white dwarf is hitting it with a powerful beam of radio waves every two minutes. None of these systems are as bright or as slow as the long-period radio transients, but maybe as we find more examples, we will work out a unifying physical model that explains all of them.

On the other hand, there may be many different kinds of system that can produce long-period radio pulsations.

Either way, we’ve learned the power of expecting the unexpected—and we’ll keep scanning the skies to solve this cosmic mystery.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Image Credit: An artist’s impression of the exotic binary star system AR Scorpii / Mark Garlick/University of Warwick/ESO, CC BY

Astronomers expect to see some repeating radio signals in space, but they usually blink on and off much more quickly. The most common repeating signals come from pulsars, rotating neutron stars that emit energetic beams like lighthouses, causing them to blink on and off as they rotate towards and away from the Earth.

Pulsars slow down as they get older, and their pulses become fainter, until eventually they stop producing radio waves altogether. Our unusually slow pulsar could best be explained as a magnetar—a pulsar with exceedingly complex and powerful magnetic fields that could generate radio waves for several months before stopping.

Unfortunately, we detected the source using data gathered in 2018. By the time we analysed the data and discovered what we thought might be a magnetar it was 2020, and it was no longer producing radio waves. Without additional data, we were unable to test our magnetar theory.

Nothing New Under the Sun

Our Universe is vast, and so far every new phenomenon we’ve discovered has not been unique. We knew that if we looked again, with well-designed observations, we had a good chance of finding another long-period radio source.

So, we used the Murchison Widefield Array radio telescope in Western Australia to scan our Milky Way galaxy every three nights for several months.

We didn’t need to wait long. Almost as soon as we started looking, we found a new source, in a different part of the sky, this time repeating every 22 minutes.

At last, the moment we had been waiting for. We used every telescope we could find, across radio, X-ray, and optical light, making as many observations as possible, assuming it would not be active for long. The pulses lasted five minutes each, with gaps of 17 minutes between. Our object looked a lot like a pulsar, but spinning 1,000 times slower.

Hiding in Plain Sight

The real surprise came when we searched the oldest radio observations of this part of the sky. The Very Large Array in New Mexico, United States, has the longest-running archive of data. We found pulses from the source in data from every year we looked—the oldest one in an observation made in 1988.

Observing over three decades meant we could precisely time the pulses. The source is producing them like clockwork, every 1,318.1957 seconds, give or take a tenth of a millisecond.

According to our current theories, for the source to be producing radio waves, it should be slowing down. But according to the observations, it is not.

In our article in Nature, we show that the source lies “below the death line,” which is the theoretical limit of how neutron stars generate radio waves; this holds even for quite complex magnetic field models. Not only that, but if the source is a magnetar, the radio emission should only be visible for a few months to years—not 33 years and counting.

So when we tried to solve one problem, we accidentally created another. What are these mysterious repeating radio sources?

What About ET?

Of course, it’s very tempting at this point to reach to extraterrestrial intelligence as an option. The same thing happened when pulsars were discovered: astrophysicist Jocelyn Bell Burnell and her colleagues, who found the first pulsar, nicknamed it “LGM 1,” for “Little Green Men 1.”

But as soon as Bell and her colleagues made further detections, they knew it could not be aliens. It would be incredibly unlikely for so many similar signals to be coming from so many different parts of the sky.

The pulses, similar to those of our source, contained no information, just “noise” across all frequencies, just like natural radio sources. Also, the energy requirements to emit a signal at all frequencies are staggering: you need to use, well, a neutron star.

While it’s tempting to try to explain a new phenomenon this way, it’s a bit of a cop out. It doesn’t encourage us to keep thinking, observing, and testing new ideas. I call it the “aliens of the gaps” approach.

Fortunately, this source is still active, so anyone in the world can observe it. Perhaps with creative follow-up observations, and more analysis, we’ll be able to solve this new cosmic mystery.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Image Credit: The International Centre for Radio Astronomy Research

It was the first lockdown of 2021 in Perth, and we were all working from home. And when astronomers look for something to distract themselves from looming existential dread, there’s nothing better than a new cosmic mystery.

In 2020 I gave an undergraduate student, Tyrone O’Doherty, a fun project: look for radio sources that are changing in a large radio survey I’m leading.

By the end of the year he’d found a particularly unusual source that was visible in data from early 2018, but had disappeared within a few months. The source was named GLEAM-X J162759.5-523504, after the survey it was found in and its position.

Sources that appear and disappear are called “radio transients” and are usually a sign of extreme physics at play.

The Mystery Begins

Earlier this year I started investigating the source, expecting it to be something we knew about; something that would change slowly over months and perhaps point to an exploded star, or a big collision in space.

To understand the physics, I wanted to measure how the source’s brightness relates to its frequency (in the electromagnetic spectrum). So I looked at observations of the same location, taken at different frequencies, before and after the detection, and it wasn’t there.

I was disappointed, as spurious signals do crop up occasionally due to telescope calibration errors, Earth’s ionosphere reflecting TV signals, or aircraft and satellites streaking overhead.

So I looked at more data. And in an observation taken 18 minutes later, there the source was again, in exactly the same place and at exactly the same frequency—like nothing astronomers had ever seen before.

At this point I broke out in a cold sweat. There is a worldwide research effort searching for repeating cosmic radio signals transmitted at a single frequency. It’s called the Search for Extra-Terrestrial Intelligence. Was this the moment we finally found that the truth is … out there?

The Plot Thickens

I rapidly downloaded more data and posted updates on Slack. This source was incredibly bright. It was outshining everything else in the observation, which is nothing to sniff at.

The brightest radio sources are supermassive black holes flaring huge jets of matter into space at nearly the speed of light. What had we found that could possibly be brighter than that?

Colleagues were beginning to take notice, posting: “It’s repeating too slowly to be a pulsar. But it’s too bright for a flare star. What is this? (alien emoji icon)???”

Within a few hours, I breathed a sigh of relief: I had detected the source across a wide range of frequencies, so the power it would take to generate it could only come from a natural source; not artificial (and not aliens)!

Just like pulsars (highly magnetized rotating neutron stars that beam out radio waves from their poles) the radio waves repeated like clockwork about three times per hour. In fact, I could predict when they would appear to an accuracy of one ten-thousandth of a second.

So I turned to our enormous data archive: 40 petabytes of radio astronomy data recorded by the Murchison Widefield Array in Western Australia, during its eight years of operation. Using powerful supercomputers, I searched hundreds of observations and picked up 70 more detections spanning three months in 2018, but none before or after.

The amazing thing about radio transients is that if you have enough frequency coverage, you can work out how far away they are. This is because lower radio frequencies arrive slightly later than higher ones depending on how much space they’ve traveled through.

Our new discovery lies about 4,000 light years away—very distant, but still in our galactic backyard.

We also found the radio pulses were almost completely polarized. In astrophysics this usually means their source is a strong magnetic field. The pulses were also changing shape in just half a second, so the source has to be less than half a light second across, much smaller than our sun.

Sharing the result with colleagues across the world, everyone was excited, but no one knew for sure what it was.

The Jury Is Still Out

There were two leading explanations for this compact, rotating, and highly magnetic astrophysical object: a white dwarf, or a neutron star. These remain after stars run out of fuel and collapse, generating magnetic fields billions to quintillions times stronger than our sun’s.

And while we’ve never found a neutron star that behaves quite this way, theorists have predicted such objects, called an “ultra-long period magnetars”, could exist. Even so, no one expected one could be so bright.

This is the first time we’ve ever seen a radio source that repeats every 20 minutes. But maybe the reason we never saw one before is that we weren’t looking.

When I first started trying to understand this source, I was biased by my expectations: transient radio sources either change quickly like pulsars, or slowly like the fading remnants of a supernova.

I wasn’t looking for sources repeating at 18-minute intervals, an unusual period for any known class of object. Nor was I searching for something that would appear for a few months and then disappear forever. No one was.

As astronomers build new telescopes that will collect vast quantities of data, it’s vital we keep our minds, and our search techniques, open to unexpected possibilities. The universe is full of wonders, should we only choose to look.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Image Credit: Artist visualization, author provided