The Earth is under constant barrage by radio signals from space. Typically they just come from mundane sources like our own satellites, your 'garden variety' stars, and even more exotic things like 'rotating neutron stars'. However, astronomers have found one type of signal whose source has, so far, remained a mystery.
No, I'm not talking about the 'Wow! signal' (although that's a great one).
For us, this story starts in 2007, when a team of astronomers reported finding an incredibly powerful and incredibly short radio burst, from a sky survey done back in 2001. It had flashed very 'bright' to their radio telescope for about 5 milliseconds and then it was gone. The pulse itself wasn't the first thing that struck them as odd, though; it was that the pulse didn't repeat. Looking at it a bit closer, they found that it got even stranger, because it came from a point in the sky that suggested it originated from outside our galaxy. In fact, they found that whatever sent forth this blast of radio waves was roughly 3 billion light years away, so not even in our galactic neighbourhood (for comparison, the Andromeda Galaxy is 'only' 2.5 million light years away).
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This mysterious radio signal became known as the 'Lorimer burst', named after National Radio Astronomy Observatory astrophysicist Duncan Lorimer, who led the team that discovered and studied the burst.
Now, fast-forward to today, and astronomers have published another study that examines four more of these signals they detected over the last four years. That's not the only ones they've seen, though. According to ABC Science, the astronomers set up a permanent detector that sees one of these bursts every 10 days or so. Their research only covers four of them because the radio telescope can only cover a very small area of the sky at any time.
These bursts are all from single events (as opposed to the same object emitting repeated bursts), that happened billions of light years away (so billions of years ago), but their source is still a mystery.
They're definitely caused by some kind of cataclysmic astronomical event — maybe a massive supernova that forms a black hole or two neutron stars colliding, but the leading contender is something that may be even more strange — a magnetic neutron star, or 'magnetar'.
A neutron star is what's left over when a star that's many times bigger than our Sun dies. This stellar 'remnant' is typically about 20 kilometres in diameter, and so dense that a mere thimble-full of its matter would weigh over 100 million tons. Some neutron stars rotate very quickly (on the order of milliseconds per rotation) and put off beams of radiation from their poles, a bit like a lighthouse shining a beacon across space. These are called 'pulsars'.
A magnetar is a neutron star that rotates much slower ('only' once every few seconds) and has incredibly powerful magnetic fields. For comparison, the strength of our own Sun's magnetic field is measured in micro-Teslas — millionths of a unit of magnetic field strength. A magnetar's magnetic field strength is measured in hundreds of millions to hundreds of billions of Telsas.
"These are the most magnetic stars in the universe, and every now and then they have very large explosions on them, which may be a good candidate for the origin of these things," said Prof. Matthew Bailes, the director of the Swinburne Centre for Astrophysics and Supercomputing, in Melbourne, Australia, according to ABC Science.
"They can give off as much energy as the Sun does for 300,000 years, in just a millisecond."
It's this potential for such incredibly powerful bursts of energy that puts them at the top of the list of suspects. Anything weaker and the signal wouldn't survive crossing intergalactic space to reach us.
Another exceptional thing about these bursts is that they carry information along with them about the universe.
"Every time the radio waves go past an electron, the electron's presence gets encoded in the burst of photons," Bailes told ABC Science.
"We can use this to count how many electrons there are between us and half way across the universe, which is incredibly exciting because almost all of the normal atoms are actually not in galaxies, they're just sitting around in the intergalactic medium."
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Astronomers have used the beams of pulsars to tell us about what lies between the pulsar and us, but in this case, it may be one mystery helping us to solve another mystery. We may not know where these bursts are coming from, but the bursts themselves will be able to tell us a lot about the universe we live in.
"We are still not sure about what makes up the space between galaxies, so we will be able to use these radio bursts like probes in order to understand more about some of the missing matter in the Universe," said study co-author Dr. Ben Stappers, from Manchester’s School of Physics and Astronomy, according to a news release. "We are now starting to use Parkes and other telescopes, like the Lovell Telescope of the University of Manchester, to look for these bursts in real time."