Astronomers looking for neutron stars said they found mysterious intergalactic radio bursts in archived sky sweeps conducted by a radio telescope in Australia.

Study leader Dan Thornton of England's University of Manchester and Australia's Commonwealth Scientific and Industrial Research Organization said the findings pointed to some extreme events involving large amounts of mass or energy as the source of the radio bursts.

"A single burst of radio emission of unknown origin was detected outside our Galaxy about six years ago but no one was certain what it was or even if it was real, so we have spent the last four years searching for more of these explosive, short-duration radio bursts," he said.

The study found four more such bursts in archival data, he said in a Manchester release Friday, "removing any doubt they are real."

Unlike most cosmic radio signals that originate in the Milky Way or a nearby neighbor galaxy, these four seem to have come from far beyond in the universe, the researchers said.

Co-author Matthew Bailes of the Swinburne University of Technology in Melbourne, said he thinks the origin of the explosive bursts may be magnetic neutron stars, known as "magnetars."

"Magnetars can give off more energy in a millisecond than our sun does in 300,000 years and are a leading candidate for the burst," he said.

Some researchers have suggested other phenomena that could trigger such bursts, including colliding magnetars, evaporating black holes or gamma ray bursts from a supernova.

Whatever is happening is probably a relatively common phenomenon, although it has proved difficult to detect, astronomers said.

Extrapolating from the research, there could be as many as 10,000 similar high-energy millisecond radio bursts happening across the sky every day, they said.

"It is still early days for identifying the astrophysical origins of such common but (so far) rarely detected events," Cornell University astronomer James Cordes wrote in the journal Science.

They could be from an entirely new type of high-energy astrophysical event, he said.