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White dwarf star blast: a new form of ‘cosmic particle accelerator’?

As the white dwarf of the AR Scorpii spins, it beams particles close to the speed of light to its companion star, causing the system to pulse with light.

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    This artist’s impression shows the strange object AR Scorpii. In this unique double star, a rapidly spinning white dwarf star (r.) powers electrons up to almost the speed of light. These high energy particles release blasts of radiation that lash the companion red dwarf star (l.) and cause the entire system to pulse dramatically every 1.97 minutes with radiation ranging from the ultraviolet to radio.
    Courtesy of M. Garlick/University of Warwick/ESO
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Although the night sky may at times seem painted still, many stars are constantly pulsing. The brightness of these variable stars changes over time due to a variety of possible processes: expansion and shrinking, eclipsing, even the sharing of mass. That is why members of the American Association of Variable Star Observers (AAVSO) dedicate their curiosity to variable stars. There’s still so much astronomers don’t know.

That curiosity is why a once little-known binary star in the Scorpius constellation is now big news. When Tom Marsh, lead author of a study published in Nature on Thursday and leader of the Astronomy & Astrophysics group in the department of physics at the University of Warwick, England, was first sent observations about AR Scorpii, the light curve didn’t look very unusual to him.

But when he saw the large variation in brightness in the light curve, he “realized there was something odd about it,” Professor Marsh tells The Christian Science Monitor in a phone interview. “That’s what made me decide to look at it with a high-speed camera”: a telescope with a four-meter aperture. 

Once Marsh looked at the AR Scorpii under the telescope, he realized it was something entirely different from anything he’d seen before. When the star was first discovered in 1971, it was classified as a lone variable star that expands and contracts, and was promptly forgotten.

Upon closer observation, Marsh and his colleagues found that it was actually a binary star made up of both a white dwarf, the collapsed remains of a star that ran out of fuel, and a red dwarf, which is similar to Earth’s sun, but smaller.

In an 'Aha!' moment, the researchers realized they were witnessing the cooler red dwarf moving away from them and coming toward them as it orbited the white dwarf.

The white dwarf, however, is the "star" of the system. “It’s got the most incredibly strong pulsations that I’ve ever seen in any star. It can get four or five times stronger within 30 seconds and then fade away again and keep doing that.”

Marsh calls it a “new type of cosmic particle accelerator.”

What's more, it emits light at almost all wavelengths, Marsh says. “It even pulses in the radio.” There is only one other similar star that emits in the radio, but it doesn’t pulse. Normally, radio waves are emitted in a large volume and quickly. These radio waves, however, are coming from energetic electrons that are traveling near the speed of light.

The white dwarf is spinning very quickly and has a strong magnetic field, stronger than Earth’s, that may accelerate electrons around its poles, Marsh says. It’s “like a very extreme aurora in a way.”

Because the magnetic pulse is closer to the equator of the white dwarf than is the case on Earth, the poles go in and out of view. “It’s really like a lighthouse shining around the system,” Marsh says.

“I don’t think we have it sussed out yet by a long shot,” Marsh says.

Next steps include looking at data from an X-ray satellite to see if the binary star is also emitting X-ray wavelengths. This data will help the researchers understand whether the wavelengths are actually coming directly from the white dwarf, Marsh says, even though he says it would be surprising if they weren’t.

Marsh would also like to learn how the white dwarf accelerates its particles near light speed. “We really at the moment don’t know how it’s doing that.” More research into this could solve the 50-year-old puzzle of how faster and more compact neutron stars, or pulsars, emit their radio radiation.

He’s also planning on collaborating with amateur astronomers again to follow the star continuously around the world.

"We've known pulsing neutron stars for nearly fifty years, and some theories predicted white dwarfs could show similar behaviour,” Boris Gänsicke co-author and professor of physics at the University of Warwick says in the European Southern Observatory (ESO) press release. “It's very exciting that we have discovered such a system, and it has been a fantastic example of amateur astronomers and academics working together.”

It’s been “hiding in plain sight for 45 years,” Marsh says. “I’m glad we found it.”

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