In a case of cosmic resuscitation, ESA's Integral X-ray and gamma-ray space observatory has, for the first time, seen a red giant star revive its dead neighbor. An X-ray flare first detected on August 13, 2017 toward the center of our galaxy prompted observations that revealed a once dead, slow-spinning neutron star becoming active again as it feeds on puffs of material ejected from its ancient, giant companion.

Neutron stars are one of the more peculiar endpoints of stellar evolution. These are the remnants of stars 25 to 30 times the mass of the Sun that eat up their nuclear fuel at a fantastic rate, then collapse in on themselves in an implosion that turns the star into a supernova. As the outer layers of the star are blasted away in an explosion that outshines an entire galaxy, all that is left behind is the star's 10-km-wide (6-mi) core called a neutron star.

This is because the inside of the core is now so dense that the electrons are squeezed into the protons, turning them into neutrons and transforming the matter of the star into neutronium. This is so dense that a single teaspoon of neutronium weighs two billion tonnes.

Neutron stars are best known to us as pulsars. First observed on November 28, 1967 by radio astronomer Jocelyn Bell Burnell, a pulsar is a neutron star that has turned into an energetic beacon flashing radio waves and X-rays out into space. Though a neutron star is a dead star, it has a powerful magnetic field, and when a star contracts into a neutron star, the conservation of angular momentum causes it to spin at a very high speed.

If the neutron star is part of a binary system with another active star, it may start to draw in matter from its companion. This collects on the surface of the neutron star, and when enough has built up a nuclear reaction occurs and there's massive flare of radio waves and X-rays, which is contained by the magnetic field except at the poles, where it shoots out as a pair of beams. The result is a pulsar, which flashes on and off so regularly that the first one was initially suspected as being a radio transmitter from an alien civilization.

The neutron star found by the ESA's unmanned Integral space observatory is strongly magnetized, but has a very slow rotation of once every two hours according to further observation by ESA's XMM-Newton and NASA's NuSTAR space telescopes. Even odder is its red giant companion star, which is the endpoint of the life of stars that are up to eight times as massive as the Sun. Finding a pulsar linked with another star is relatively common, but a neutron star linked with a red giant is called a "symbiotic X-ray binary" and is very rare – only about 10 are known. Even more interesting, this neutron star is just starting to feed on the other.

"Integral caught a unique moment in the birth of a rare binary system," says Enrico Bozzo from University of Geneva and lead author of a paper detailed the discovery. "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."

Measurements of the neutron star's magnetic field is very strong, indicating that it's very young. This raises a few questions because red giants are very old stars, so the neutron star couldn't have been produced by a supernovae explosion, which would have destroyed the companion.

"These objects are puzzling," says Enrico. "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."

According to the ESA team, the neutron star is only receiving puffs of gas from the giant, but over time the flow will become regular, slowing down the neutron star and causing it to regularly emit 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."

The research will be published in Astronomy & Astrophysics.

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