Astronomers have discovered a mystery object in the sky that was sending out a beam of radiation every 20 minutes - an unusual pattern that has not been seen before.
But the scientists said it was not aliens sending signals to Earth. It was probably a magnetar - a neutron star with an extremely strong magnetic field - or the collapsed remnant of a star known as a white dwarf.
The team in Australia and China published their findings in the peer-reviewed journal Nature on Thursday. Their analysis is based on observations taken by the Murchison Widefield Array (MWA), a low-frequency radio telescope located in Western Australia, far away from cities where phones and other devices can cause radio frequency interference.
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Lead researcher Natasha Hurley-Walker, an astrophysicist at the Curtin University node of the International Centre for Radio Astronomy Research, said the object was about 4,000 light years away, which she described as "quite close to us" and "in our galactic backyard".
"A repeating radio signal from space - I was concerned that it was aliens," she said. "But what's really good about these observations is that they are over a very wide frequency range, and so are these pulses ... It is across a very wide range of frequencies, and that means it must be a natural process - this is not an artificial signal."
Astronomers call objects that turn on and off in the universe "transients". Understanding them would allow exploration of extreme physics, like the intersection between quantum mechanics and general relativity, according to Hurley-Walker.
"Radio transients come and go. Once they've appeared and then disappeared, if you don't have enough observations, you can't work out what the object was," she said.
"That's a shame because we would really like to understand what's generating these kinds of things, which often come from very high energy processes in the universe."
By going through the MWA archive of observations since 2013, the scientists found 71 pulses from this source in the period from January to March 2018.
Hurley-Walker said the periodicity of the pulses had not been observed before in astronomy. "This source appears and disappears really regularly like clockwork. It switches on, and then it switches off. And every 20 minutes, it repeats," she said.