With the power to unleash as much energy in 10 seconds as the Sun does in its entire lifetime, gamma-ray bursts (GRBs) are the most energetic events in the universe, second only to the Big Bang itself. Given how sudden they are, we don't usually get any warning, but now astronomers have spotted the telltale signs of a star system that could blow its lid any moment – and it's far too close to Earth for comfort.
GRBs have a number of possible sources, depending on the length of the event. They can be emitted when a star goes supernova, or from the collision of two neutron stars. Most of the time though the source isn't clear, since they're usually discovered after the fact as a bright burst of light and electromagnetic radiation, and often occur very far away, beyond our galaxy.
This new discovery is significant on both of those fronts. One of the two stars in this strange system appears to be a gamma-ray burst progenitor – meaning it's a possible future source of a gamma-ray burst – and it's far closer to Earth than usual. At a distance of a mere 8,000 light-years, the system is well within the Milky Way.
"This system is likely the first of its kind ever discovered in our own galaxy," says Benjamin Pope, an author of the study. "It was not expected such a system would be found in our galaxy – only in younger galaxies much further away. Given its brightness, it is surprising it was not discovered a lot sooner."
The stars belong to a group known as Wolf-Rayet stars, which are rare, hot and luminous. What makes this pair particularly eye-catching is the "pinwheel" of dust and gas that's swirling around them. This is created by the extremely fast stellar winds generated as one of the stars throws off its outer layers of mass during its death throes.
"We knew immediately we had found something quite exceptional: the luminosity across the spectrum from the radio to the infrared was off the charts," says Joe Callingham, lead author of the study. "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."
Naming the system after a mythological embodiment of chaos is pretty apt – on closer inspection, Apep is even weirder still. According to the data gathered by the team, the stellar winds, which were clocked at about 12 million km/h (7.5 million mph), are expanding 10 times faster than the dust in the pinwheel. That suggests the star is spinning so fast it might be headed for an explosive end in the near future.
"Apep's dust pinwheel moves much slower than the wind in the system," says Callingham. "One way this can occur is if one of the massive stars is rotating so quickly that it is nearly tearing itself apart. Such a rotation means that when it runs out of fuel and begins to explode as a supernova, it will collapse at the poles before the equator, producing a gamma-ray burst."
And that's where Apep's proximity to us is a little worrying. Although 8,000 light-years sounds like a fair old hike, it's a blip on the cosmic scale, and a gamma-ray burst that close could have major effects here on Earth. These bursts, occurring very close by, have been blamed for mass extinction events in the distant past, as they strip away ozone from the Earth's atmosphere and leave life vulnerable to increased UV exposure from the Sun.
More recently, some of the brightest GRBs in recorded history were detected in 1998 and 2004, which were visible by detectors and satellites as extremely bright flashes of light lasting just milliseconds. In both cases, disruptions were seen in the Earth's ionosphere, albeit nothing too serious. And these two events originated 20,000 and 50,000 light-years away – much further away than Apep.
Thankfully, there's some good news. For starters, "soon" is a relative term when you're talking about space, so Apep might stretch its death scene out for hundreds of thousands of years yet. Even if it does, scientists don't know for sure that it will produce a gamma-ray burst, and even if it does, Apep seems to be pointing away from Earth so we shouldn't cop the brunt of it.
"Ultimately, we can't be certain what the future has in store for Apep," says Peter Tuthill, an author of the study. "The system might slow down enough so it explodes as a normal supernova rather than a gamma-ray burst. However, in the meantime, it is providing astronomers a ringside seat into beautiful and dangerous physics that we have not seen before in our galaxy."
The research was published in the journal Nature Astronomy.
Sources: University of Sydney, New York University
