Astronomers Finally Solve the Mystery of Strange Repeating Radio Bursts From Space

Exotic Binary Star System AR Scorpii Artist’s Impression — This artist’s impression shows the strange object AR Scorpii. In this unique double star, a rapidly spinning white dwarf star (right) powers electrons up to almost the speed of light. These high-energy particles release blasts of radiation that lash the companion red dwarf star (left) and cause the entire system to pulse dramatically every 1.97 minutes with radiation ranging from the ultraviolet to radio. Credit: M. Garlick, University of Warwick, ESO, edited

Recent discoveries reveal that bursts of slow pulsing radio waves originate from a binary star system consisting of a red dwarf and a <span class="glossaryLink" aria-describedby="tt" data-cmtooltip="

white dwarf

A white dwarf star is the remnant of star that has exhausted its nuclear fuel, but it lacks the mass to become a neutron star. A typical white dwarf is only slightly bigger than Earth, yet it is 200,000 times as dense.

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These findings challenge current <span class="glossaryLink" aria-describedby="tt" data-cmtooltip="

pulsar

First observed at radio frequencies, a pulsar is a rotating neutron star that emits regular pulses of radiation. Astronomers developed three categories for pulsars: accretion-powered pulsars, rotation-powered pulsars, and nuclear-powered pulsars; and have since observed them at X-ray, optical, and gamma-ray energies.

” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>pulsar theories and indicate a wider variety of stellar systems may emit similar signals.

Radio Wave Mysteries

Since 2022, astronomers have been puzzled by bursts of intense radio waves from deep space that slowly repeat at regular intervals. These signals, unlike anything seen before, defy conventional understanding of how such cosmic phenomena work.

For the first time, new research has traced one of these mysterious signals back to its source: a common, lightweight star called a red dwarf. It appears to be in a binary orbit with a white dwarf — the dense core left behind when a star similar to our Sun dies in a dramatic explosion.

Discovery of a New Cosmic Phenomenon

In 2022, our team made a remarkable discovery: periodic radio pulses from space that repeated every 18 minutes. The bursts were incredibly bright, outshining anything nearby. After three months of flashing, they vanished without a trace.

We know that some repeating radio signals come from neutron stars known as radio pulsars. These stars spin rapidly, often rotating once per second or even faster, sending out beams of radio waves like a lighthouse. However, based on what we currently understand, a pulsar spinning as slowly as once every 18 minutes shouldn’t be able to produce radio waves at all.

This unexpected finding suggested we might be dealing with an entirely new type of astronomical phenomenon — or that our understanding of how pulsars emit radio waves needs to be reconsidered.

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

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

Murchison Widefield Array Element — One element of the Murchison Widefield Array, a radio telescope in Western Australia that observes the sky at low radio frequencies. Credit: ICRAR / Curtin University

Searching the Outskirts of the Galaxy

Until now, every one of these sources has been found deep in the heart of the <span class="glossaryLink" aria-describedby="tt" data-cmtooltip="

Milky Way

The Milky Way is the galaxy that contains our Solar System and is part of the Local Group of galaxies. It is a barred spiral galaxy that contains an estimated 100-400 billion stars and has a diameter between 150,000 and 200,000 light-years. The name "Milky Way" comes from the appearance of the galaxy from Earth as a faint band of light that stretches across the night sky, resembling spilled milk.

” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>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.

Source of Radio Waves MWA MeerKAT — The source of the radio waves, as seen by the MWA at low resolution (magenta circle) and MeerKAT at high resolution (cyan circle). The white circles are all stars in our own Galaxy. Credit: Hurley-Walker et al. 2024 / Astrophysical Journal Letters

Pinpointing the Source of Radio Waves

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% 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 <span class="glossaryLink" aria-describedby="tt" data-cmtooltip="

neutron star

A neutron star is the collapsed core of a large (between 10 and 29 solar masses) star. Neutron stars are the smallest and densest stars known to exist. Though neutron stars typically have a radius on the order of just 10 – 20 kilometers (6 – 12 miles), they can have masses of about 1.3 – 2.5 that of the Sun.

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black hole

A black hole is a place in space where the gravitational field is so strong that not even light can escape it. Astronomers classify black holes into three categories by size: miniature, stellar, and supermassive black holes. Miniature black holes could have a mass smaller than our Sun and supermassive black holes could have a mass equivalent to billions of our Sun.

” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]” tabindex=”0″ role=”link”>black hole, the explosion that created it would have been so large it should have disrupted the orbit.

An artist’s impression of the AR Sco system: a binary red dwarf and white dwarf that interact to produce radio emission.

A Binary System’s Role in Radio Wave Production

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.

Continuing the Quest for Answers

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.

Written by Natasha Hurley-Walker, Radio Astronomer, Curtin University.

Adapted from an article originally published in The Conversation.

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