Slowly repeating bursts of intense radio waves coming from space have puzzled astronomers since they were discovered in 2022.
In new research, we have for the first time tracked one of these pulsed signals back to its source: a common type of faint star called a red dwarf, possibly in a binary orbit with a white dwarf, the core of another star that exploded long ago.
The slowly pulsating mystery
In 2022, our team made an amazing discovery: periodic radio pulses emanating from space that repeat every 18 minutes. The pulses outshine everything nearby, shine brilliantly for up to three months, then disappear.
We know that some of the repeating radio signals come from a type of neutron star called a radio pulsar, which spins rapidly (typically once a second or even faster), shooting out radio waves like a lighthouse. The problem is, our current theories say that pulsars that spin only once every 18 minutes shouldn’t make radio waves.
So we thought our 2022 discovery might point to new and exciting physics – or help explain how pulsars emit radiation, which is still not very well understood despite 50 years of research.
More slowly brightening radio sources have been discovered since then. There are now about ten known “long-period radio transients”.
However, simply finding more is not enough to solve the mystery.
Exploring the outskirts of the galaxy
So far, every single one of these sources has been found deep in the heart of the Milky Way.
This makes it very difficult to work out what kind of star or object generates 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 at all.
So, we began a campaign of scanning the skies with the Murchison Widefield Array radio telescope in Western Australia, which can observe 1,000 square degrees of sky every minute. Csanád Horváth, a graduate student at Curtin University, processed data covering half of the sky, looking for these elusive signals in more sparsely populated regions of the galaxy.
And sure enough, we found a new source! Called GLEAM-X J0704-37, it generates minute-long pulses of radio waves, just like other long-duration radio transients. However, these pulses repeat only once every 2.9 hours, making it the slowest long-duration radio transient ever found.
Where the radio waves are coming from
We made 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: they were coming from a red dwarf star. These stars are incredibly common, making up 70% of the stars in the galaxy, but they are so faint that not a single one is visible to the naked eye.
By combining historical observations from the Murchison Widefield Array and new MeerKAT monitoring data, we found that the pulses come a little earlier and a little later in a repeating pattern. This likely indicates that the radio emitter is not the red dwarf itself, but an invisible 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 a white dwarf, which is the end point 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 massive that it would have disrupted the orbit.
It takes two to tango
So how do a red dwarf and a white dwarf produce radio signals?
The red dwarf probably generates a stellar wind of charged particles, just as our Sun does. When the wind collides with the white dwarf’s magnetic field, it is accelerated, producing radio waves.
This may be similar to how the Sun’s stellar winds combine with the Earth’s magnetic field to produce beautiful auroras and low-frequency radio waves.