A star bursts back into life

A slowly spinning dead star sucks up enough matter from its partner to re-ignite its long-extinguished fire

Artist’s impression depicting winds from a red giant star impacting a neutron star to create prolonged X-ray emission.

Truly Curious

A turbid, swollen red giant star has been seen infusing new life of sorts into its slow-spinning companion, a neutron star, the core of a dying sun that was feeding off it.

The X-ray flare was first detected by Integral, the European Space Agency’s space observatory on August 13, 2017. It appeared to come from an unknown source in the direction of the crowded center of our Milky Way. The sudden detection triggered a slew of follow-up observations in the following weeks to pin down the culprit.

The observations revealed a strongly magnetized and slowly rotating neutron star that had likely just begun to feed on material from a neighboring red giant star. The researchers could only observe it until October 27 but they could compare it with other observations by other stations during and long before the event.

Stars the mass of our Sun, and up to eight times more massive, evolve into red giants towards the end of their lives. Their outer layers puff up and expand millions of miles, their dusty, gassy shells blown away from the central star in relatively slow winds up to few hundreds of miles per second.

Even larger stars, up to 25–30 times more massive than the Sun, race through their fuel and explode in a supernova, sometimes leaving behind a spinning stellar corpse with a strong magnetic field, known as a neutron star. This tiny core packs the mass of nearly one and a half Suns into a sphere about six miles across, making them among the densest celestial objects known.

It is not uncommon to find stars paired together, but the new system of a neutron star and red giant is a particularly rare breed known as a “symbiotic X-ray binary,” with no more than 10 known.

“Integral caught a unique moment in the birth of a rare binary system,” Enrico Bozzo from the University of Geneva and lead author of the paper that describes the discovery, said in a ESA 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.”

The pairing is certainly peculiar. ESA’s XMM-Newton and NASA’s NuSTAR space telescopes showed that the neutron star spins almost every two hours – very slow compared with other neutron stars, which can spin up to many times per second. Then, the first measurement of the magnetic field of such a neutron star revealed it to be surprisingly strong.

A strong magnetic field typically points to a young neutron star – the magnetic field is thought to fade over time – while a red giant is much older, making this a bizarre couple.

“These objects are puzzling,” says Enrico. “It might be that either the neutron star’s 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.” That is, it could have been the remnant of a small star that fell into the company of the red giant that it fed off until it became large enough to become a neutron star and then flame back into life.

With a young neutron star and an old red giant, at some point the winds traveling from the puffed-up giant will begin to rain on to the smaller star, slowing its spin and emitting X-rays.

“We haven’t seen this object before in the past 15 years of our observations with Integral, so we believe we saw the X-rays turning on for the first time,” says Erik Kuulkers, ESA’s Integral project scientist. “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.”

Based on material provided by the European Space Agency/

IGR J17329-2731: the birth of a symbiotic X-ray binary,” by E Bozzo et al has been accepted for publication in Astronomy & Astrophysics.

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