Mysterious signals keep coming from space. We have found their ‘Rosetta stone’

A pair of stars spiralling around each other. That’s the origin of a new source of repeating radio bursts we’ve detected, called ASKAP J1745.

In recent years, astronomers have been puzzling over mysterious bursts of radio signals, known as long-period transients because of how slowly they repeat. They were first discovered by chance with telescopes scanning large chunks of the sky.

To date, astronomers have only found a dozen of these weird sources, and we’re still trying to understand exactly what they are.

In a new study published today in Nature Astronomy, we describe a first-of-its-kind detection – both radio and X-ray bursts repeating with each orbit.

ASKAP J1745 is exciting because we’ve figured out what it is, unlike 10 of the 12 known long-period transients. Even better, we were able to detect it with a bunch of different telescopes that observe all different kinds of light.

Bearing the same message in three forms of writing, the famous Rosetta stone once helped scholars decipher ancient Egyptian hieroglyphs. Similarly, this extra information we found about ASKAP J1745 will help astronomers better understand the mystery of all long-period transients.

What do long-period radio transients look like?

Long-period transients are things in space that produce bright, repeating bursts of light at radio wavelengths. Little is known about the origins of most long-period transients. In addition, many have been discovered close to the dusty region in the middle of our galaxy, so it can be hard to see them with visible-light telescopes.

Even with just a dozen of these strange sources discovered so far, they seem to come in a few different shapes and sizes. Their radio bursts repeat on timescales of minutes to hours.

Some have been making regular pulses for more than 30 years, while others turn off for days at a time or go permanently radio-silent.

Where do they come from?

Astronomers initially thought long-period transients were just very slowly spinning neutron stars, called pulsars. These are the fast-rotating dense cores left after the supernova explosions of massive stars.

The first few of these radio transients discovered were repeating roughly every 20 minutes. That’s much slower than the average pulsar, which repeats every few seconds.

Furthermore, when pulsars slow down their spin, they should stop producing radio light. This means we shouldn’t see radio bursts from neutron stars rotating so slowly.

So astronomers investigated other theories involving white dwarfs – the slowly cooling dead centres of less massive stars. And recently we discovered some long-period transients in binary systems (two stars in a close orbit) with evidence of both a white dwarf and a lower-mass red dwarf star.

The discovery of ASKAP J1745

ASKAP J1745 is a new long-period radio transient we found with the ASKAP radio telescope, owned and operated by CSIRO, Australia’s national science agency. It’s the first one of these strange sources that we’ve identified as a “cataclysmic variable”.

Cataclysmic variables are systems with two stars – one of them a white dwarf – that orbit each other closely enough to interact. If the stars are close enough, the white dwarf’s gravity can pull (or “accrete”) material from the other star. That’s why these systems are also known as accreting white dwarf binaries.

Another long-period radio transient was recently discovered with X-ray bursts, repeating with the same regularity as the radio. However, the origin of the bursts and their shared timing remained unclear.

Now, for the first time, we have combined observations from radio, X-ray and optical telescopes to find that ASKAP J1745 produces both X-ray and radio bursts with each orbit of its two stars.

In these rapidly orbiting systems, the X-ray light is thought to come from the material heating up as it streams onto the white dwarf.

The bright radio bursts were a bit more of a mystery. But knowing that this is an accreting binary system helped us figure things out.

The type of pulsed radio light we detected is typically caused by energetic particles interacting with strong magnetic fields. Here, we have the perfect combination: two stars with strong magnetic fields (typically thousands of times stronger than an MRI machine), with charged particles flowing towards the white dwarf from the other star.

What this means for the future of astronomy

This discovery is unique because we have more information and at more different wavelengths than any other previous long-period transient.

Just like the Rosetta stone was key to decoding ancient Egyptian symbols, ASKAP J1745 will be key to deciphering the origins of other long-period radio transients that lack information at other wavelengths.

ASKAP J1745 is the first long-period transient showing signs of accretion across the spectrum of light – from radio waves to visible to X-rays. And this stream of charged material is a crucial ingredient for making the radio light we detect from these systems.

Exploring the mechanism that produces long-period radio bursts gives us a new laboratory to learn about extreme physics such as plasma flows and magnetic fields in conditions we can’t recreate on Earth.

We acknowledge the Wajarri Yamaji as the Traditional Owners and Native Title Holders of Inyarrimanha Ilgari Bundara, the CSIRO Murchison Radio-astronomy Observatory where ASKAP is located.