Dead star talking. Aussie astronomerunwraps a cosmic mystery.

Adam Spencer
6 hours ago
5 min read

A Sydney PhD student has traced one of space's most baffling radio signals to a white dwarf feeding off its neighbour, the clearest clue yet to a mystery that has held out for years.

(First published on my Substack where you can get #NerdNews, marvellous maths and general geekery.)

("LPT Sound System": created by the author via imagen)

Give us a wave.

Our restless, bubbling universe is awash with waves. Various processes create them. Stars detonating as supernovae. The burnt-out cores of dead stars, spinning hundreds of times a second. Superheated gas spiralling into a thirsty black hole. We can even detect the faint afterglow, still humming, from the Big Bang itself.

These waves vary in length and intensity, from the frantic, rapid-fire jitter of a gamma ray to the long, languid roll of a radio wave. They all belong to one family, the electromagnetic spectrum, and they are all, technically, light.

We catch the thin sliver our eyes evolved to read as colour. Given wavelengths can be anything from the length of a footy field, to waves shorter than the nucleus of an atom, our amazing human eyes only see a fraction of that range.

For the rest, we build incredibly powerful telescopes to peer into deep space.

In fact, talking footy, you are riding waves of all sorts every time you're at a game. Your eyes soak up light bouncing off the players. Your ears catch something different again, the roar of the crowd, carried as ripples in the air rather than light. All waves. All wonderful.

Occasionally a cosmic wave comes along that puzzles us. Where did it come from? What made it? What deep process of the cosmos is it quietly pointing to?

Well, a team at the University of Sydney just cracked one that had stumped the best minds in the field for years. And it is a cracker of a story.

LPT sound system.

Meet the new noise in town. A strange new class of celestial signals that astronomers call long-period radio transients, or LPTs.

LPTs are bursts of radio waves that flare for a minute or two, then switch off, only to return with the regularity of a metronome. Some LPTs repeat every few minutes. Others come back every few hours. And in total, only around a dozen have ever been spotted in the Milky Way.

Minutes? Hours? This is what puzzles astronomers.

You see, these timescales were far too slow to be the typical culprit that produces repeated radio pulses. A spinning neutron star repeats on time scales of a few seconds, even fractions of seconds for the really fast spinners. The LPT at issue here is sending out its pulse every hour and a bit. Huh?

Since the detection of the first LPT, the gorgeously named ‘Galactic Burper’ in 2005, astronomers have been stumped. According to standard physics models, anything ticking that slowly shouldn't be sending radio waves at all, at least not a single spinning star.

It takes two to space tango.

Writing in Nature Astronomy, PhD candidate Kovi Rose has blown the case wide open. It all starts with a white dwarf bearing the distinctly ungorgeous name of ASKAP J1745-5051. Space nerds will spot ‘ASKAP’ as the CSIRO radio telescope in outback Western Australia that first spotted it.

When a star like our Sun burns itself out, it will collapse to a ball roughly the size of Earth. This incredibly dense clump of Sun-like mass in Earth-like volume (we’re talking a teaspoon of it weighs tonnes!) is a white dwarf.

Crucially this white dwarf has company. It orbits a red dwarf about a tenth the Sun's mass. The two of them are whipping around each other in just over an hour, which is pretty much the same time signature as the LPT Rose had been listening to. So the signal is tied to this dance of two stars, not the spin of just one.

"For the first time we have pinpointed the origin of these signals." — Kovi Rose, University of Sydney.

Who’s a hungry white dwarf then?

These two dance partners are not equals. The white dwarf is so much more massive it rips, or accretes, gas away from the red dwarf. Systems like this are known as magnetic cataclysmic variables. While earlier LPTs had been linked to white dwarf pairs, those pairs were detached. Merely orbiting at a distance. This is the first of these radio sources we've caught in the act of gobbling up a partner’s celestial matter.

"Some similar objects had been linked to binary systems before, but this is the first one where we can clearly see both stars and the accretion process in action." — Tara Murphy, University of Sydney.

X marks the spot.

So what is ASKAP actually picking up?

To be completely honest, we don't know but Rose and the team might have gotten us a lot close to finding out.

Importantly, it is NOT sound. Despite the R-word, a 'radio' telescope is a dish catching light your eyes can't see. It is not a deep space version of what you have in your car.

The bursts seem to be flung out where the two stars' magnetic fields meet and tangle. A tight pulse of radio energy thrown off once an orbit.

("Cosmic Tango": created by the author via imagen)

Exactly how a system like this fires off such bright, regularly repeating pulses is still an open question.

But what sets the rhythm of the system is now nailed down.

And the clincher was that the same beat showed up in X-rays too, caught by the Einstein Probe satellite. Orbit, radio pulse and X-ray flash all ticking together is what convinced the team the two stars circling each other is what drives the signal.

"These systems are natural laboratories. They allow us to test our understanding of how matter behaves in strong magnetic fields and under intense gravitational forces." —Kovi Rose, University of Sydney.

Rose’s Rosetta.

Giving props to the famous ancient tablet that let us decode until them impenetrable Egyptian hieroglyphs, Rose's team calls the object a Rosetta stone.

With this LPT now tied to a dwarf pair, astronomers have a reference when sorting the white dwarfs from the neutron stars across this still-confusing class.

There is still a long way to go before we understand all LPTs, but some awesome Aussie scientists have done a great job here.