Mysterious energy readings from deep space that some experts have suggested could be evidence of alien life have been detected for only the second time.
The fast radio bursts (FRBs) are millisecond-long flashes of radio waves of unknown origin, and scientists have formulated several different theories as to how they might be generated.
Most believe the signals come from powerful astrophysical phenomena such as black holes or super-dense neutron stars, but others argue that they could be a result of advanced alien technology.
The last time they were detected was in 2007, when one was spotted by chance in radio astronomy data that had been collected in 2001.
In 2017, Professor Avid Loeb, from the Harvard-Smithsonian Centre for Astrophysics in the US, proposed that FRBs could be leakage from planet-sized alien transmitters.
He said that rather than being designed for communication, they would more likely be used to propel giant space ships powered by light sails, which would bounce the beams off a huge reflective sheet to provide forward thrust.
In the journal The Astrophysical Journal Letters, he said: "An artificial origin is worth contemplating and checking."
Astronomers from Canada are responsible for the latest finding, which came over a period of three weeks last summer.
Thirteen flashes were seen via a new radio telescope dubbed the Canadian Hydrogen Intensity Mapping Experiment, affectionately known as Chime.
Journal Nature reported that one of the FRBs – of which 60 total have been detected to date – was repeating.
Chime astrophysicist Dr Ingrid Stairs, from the University of British Columbia, explained its significance.
He said: "Until now, there was only one known repeating FRB. Knowing that there is another suggests that there could be more out there.
"And with more repeaters and more sources available for study, we may be able to understand these cosmic puzzles, where they're from and what causes them."
FRBs are thought to emanate from sources billions of light years away outside our galaxy, the Milky Way.
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Most of the 13 found by Chime showed signs of "scattering", which scientists said suggests they could come from powerful astrophysical objects in locations with special characteristics.
Another notable attribute of the new FRBs is their unusually low radio frequencies – coming in at 800 megahertz rather than the 1,400 megahertz of most previously detected signals.