Winds Emitted by a Red Giant Revived a Nearby Dead Neutron Star

There’s some adage out there about helping out your neighbor, right? Well, a red giant star was recently spotted lending more than just a cup of flour to its companion neutron star — it blasted it with winds that nourished the dead star back to life in a burst of X-ray light.

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The X-ray burst was first detected by the European Space Agency’s Integral space observatory in August 2017, but because of its location in the direction of the Milky Way’s center, researchers weren’t able easily identify its origin. It took weeks of observation to trace the X-ray flare back to its source: a slowly rotating neutron star orbiting a red giant star. The discovery is outlined in a paper accepted for future publication in Astronomy & Astrophysics.

Stars with masses ranging from the size of the Sun to eight times that mass turn into red giants toward the end of their life. As they age, their outer layers are slowly pushed away from the star’s center by solar winds traveling at a few hundred kilometers per second. Over time, the outer layers can expand by millions of kilometers.

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Rather than floating off into no-man’s-land like they normally would, the winds from the red giant were captured by its neighboring neutron star, which flared up in X-rays as it began accreting them.

“Integral caught a unique moment in the birth of a rare binary system,” says Enrico Bozzo, lead author of the paper, in a press release. “The red giant released a sufficiently dense slow wind to feed its neutron star companion, giving rise to high-energy emission from the dead stellar core for the first time.”
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Neutron stars typically develop when stars eight to 30 times more massive than the Sun run out of fuel and detonate in a supernova, leaving a small but incredibly massive spinning core behind. With masses of one and a half times that of the Sun squeezed into a core only about 6 miles (10 kilometers) across, neutron stars are some of the densest objects in the universe.

Neutron stars also exhibit a powerful magnetic field in the object’s early years, which is thought to fade significantly over time. When this particular neutron star’s magnetic field was measured, it was found to be quite strong, hinting that it’s still relatively young. However, its red giant companion is much older, leading researchers to wonder how the system has evolved to this point. One answer could be mass transfer, which can occur in close binary systems when material from one star is pulled in by or expands out past the other.

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“These objects are puzzling,” Enrico said. “It might be that either the neutron star magnetic field does not decay substantially with time after all, or the neutron star actually formed later in the history of the binary system. That would mean it collapsed from a white dwarf into a neutron star as a result of feeding off the red giant over a long time, rather than becoming a neutron star as a result of a more traditional supernova explosion of a short-lived massive star.”

Integral hasn’t seen anything like this in the 15 years it’s been observing, making researchers think that it’s the first massive X-ray burst associated with winds that the red giant has supplied to the neutron star. As noted by Bozzo, this type of system is incredibly rare; called a symbiotic X-ray binary, only a handful of these duos are known. Thus, studying how the unlikely pair came to be could give more insight into the different way neutron stars evolve or how magnetic fields dissipate over time.

“We’ll continue to watch how it behaves in case it is just a long ‘burp’ of winds, but so far we haven’t seen any significant changes,” said Enrico.

Regardless of how the duo formed, a good Earth-bound lesson can also be taken from this discovery: Be kind to your neighbor, because you never know when you’ll need to call in a favor.