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Doomed Star Could Fire Milky Way’s First Gamma-Ray Burst

Astronomers at the Sydney Institute for Astronomy have found a star system about 8000 light years from Earth that is the first known candidate in the Milky Way to produce a dangerous gamma-ray burst when it explodes and dies.

The system comprises a pair of scorchingly luminous Wolf-Rayet stars in the southern constellation of Norma, just beneath Scorpio’s tail. One star is on the brink of a massive supernova explosion.

The findings, published in Nature Astronomy (https://goo.gl/2eCfg3), are controversial as no gamma-ray burst has ever been detected within the Milky Way.

The Wolf-Rayet stars orbit each other every 100 years or so. Using spectroscopy, the astronomers measured the stellar winds streaming off the stars at 12 million km/h, which is about 1% the speed of light.

“When we saw the stunning dust plume coiled around these incandescent stars, we decided to name it ‘Apep’ – the monstrous serpent deity and mortal enemy of Sun god Ra from Egyptian mythology,” said Dr Joe Callingham, lead author of the study.

“When we saw the spiral dust tail we immediately knew we were dealing with a rare and special kind of nebula called a pinwheel,” said Prof Peter Tuthill, research group leader at the University of Sydney. “The curved tail is formed by the orbiting binary stars at the centre, which inject dust into the expanding wind, creating a pattern like a rotating lawn sprinkler. Because the wind expands so much, it inflates the tiny coils of dust, revealing the physics of the stars at the heart of the system.”

However, the data on the plume presented a conundrum: the stellar winds were expanding ten times faster than the dust. “It was like finding a feather caught in a hurricane just drifting along at walking pace,” Tuthill said.

Dr Benjamin Pope of New York University explained: “The key to understanding the bizarre behaviour of the wind lies in the rotation of the central stars. What we have found in the Apep system is a supernova precursor that seems to be very rapidly rotating, so fast it might be near break-up.”

Wolf-Rayet stars are massive stars at the ends of their lives; they could explode as supernovae at any time. “The rapid rotation puts Apep into a whole new class. Normal supernovae are already extreme events, but adding rotation to the mix can really throw gasoline on the fire.”

The researchers think this might be the recipe for a perfect stellar storm to produce a gamma-ray burst, which are the most extreme events in the universe after the Big Bang itself. Fortunately, Apep appears not to be aimed at Earth, because a strike by a gamma-ray burst from this proximity could strip ozone from the atmosphere, drastically increasing our exposure to UV light from the Sun.