It's ALIIIIIVE: Boffins detect slow-moving zombie star

Red giant revives a nearby corpse through the emission of cosmic wind

The European Space Agency's INTEGRAL space observatory has captured an unusual piece of stellar voodoo: the moment when a dead star was brought back to life by a nearby red giant.

The slowly rotating core of the zombie star was revived by x-ray flares from its engorged neighbour, giving rise to a high energy emission from the corpse.

Dr. Enrico Bozzo, lead author of the paper detailing the discovery (PDF), and his team first noticed the flare in data from the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) space observatory on August 13 2017 from an unknown source in the centre of the Milky Way.

The discovery triggered a slew of follow-up observations, which resulted in boffins catching the red giant red handed, so to speak.

A red giant is a star in a late evolutionary phase. Starting out a similar size to the Sun but potentially more massive, the outer layers of the star expand outwards as its fuel is exhausted. As the star cools, it takes on a red hue.

Boffins expect that a similar fate awaits our Sun, although there is some uncertainty as to what this will mean for the Earth. Whether there will be any humans around in five-six billion years’ time to witness the event is also very much open to question.

The revived zombie star is likely the remains of a star considerably more massive than the Sun, that exploded in a supernova after racing through its fuel like a carelessly tossed match in a fireworks factory.

Occasionally, a hugely dense stellar corpse is left behind, packing more than the mass of the Sun into a core less than 30km across with a strong magnetic field.

The team used data from other space observatories, ESA’s XMM-Newton and NASA’s NuSTAR, to show that the neutron star in question spins unusually slowly, only once every 2 hours, and possesses a surprisingly strong magnetic field.

The data presents boffins with a puzzle. The red giant appears to be much older than the neutron star, since this magnetic field would normally be expected to fade over time.

Scientists speculate that the neutron star actually arose from the collapse of a white dwarf, fed by the red giant over time (rather than a supernova), or that assumptions around the fading of the magnetic field need a rethink.

This particular pairing of red giant and neutron star is known as a "symbiotic X-ray binary" system, with no more than 10 known.

Boffins will continue to monitor the odd couple, in case the x-ray flare was just a "burp of winds". ®

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