Scientists studying FRBs – energy explosions from distant parts of the universe – are on to something vitally important. But what causes them?

In summer 2006, astronomer Duncan Lorimer started work on a seemingly routine piece of scientific research. He and a team of students began examining old records of sky surveys that had been carried out using the Parkes radio telescope in Australia in past years.

Lorimer was looking for observations of pulsars – highly energetic rotating neutron stars left over from supernovae explosions – that might have been missed during previous sweeps of the heavens. Pulsars are his celestial obsession, the astronomer admits, and he was keen to discover as many new ones as possible.

But what Lorimer found during that routine survey was far more dramatic – and puzzling. “We were well into carrying out our reassessment of the survey when a student came into my office one day with a data plot of a signal,” Lorimer said. “It had been recorded in 2001 and it was incredibly bright and energetic but it was also incredibly distant. It was unlike anything we had ever seen before.”

The burst was not only intensely energetic, it was also very brief, lasting only a few thousandths of a second – hence the fact it had been missed by astronomers at Parkes. As flashes go, this one had been over extremely quickly.

As to the cause of this heavenly radio micro-burst, Lorimer – who is now based at West Virginia University – was baffled. “We knew the signal came from the direction of the Magellenic clouds and we then spent more than 100 hours using telescopes to try to pinpoint its source. However, we found nothing. We couldn’t trace it to any known object in the sky. The cause of that burst was a mystery.”

Lorimer was convinced the signal came from deep space, from sources outside our galaxy. However, if it did, it would have required extraordinary energies to be detectable on Earth. As a result, some sceptical astronomers argued that his radio micro-burst must have had sources closer to home, possibly from interference from mobile phones or microwave ovens.

But when other researchers looked back at other old surveys, they found similar bursts that had also been overlooked. These signals are now known – with prosaic exactitude – as “fast radio bursts” or FRBs. All appear to be random and persist for only a few milliseconds – a fraction of a blink of an eye.

To date, just over two dozen have been detected and described. Yet scientists still cannot agree what causes them. Suggestions have included the idea that they are emitted by exploding stars, super-luminous supernovae or supermassive black holes that are ejecting material. Others argue that they are the last primal electromagnetic “screams” of stars being torn apart by black holes in a distant part of the universe or possibly the remote reverberations of engines that were powering huge interstellar spaceships driven by aliens.

Facebook Twitter Pinterest FRB 121102: in a dwarf galaxy far, far away. Photograph: Gemini Observatory

In fact, astronomers acknowledge that there are probably more theories to explain FRBs than there are actual observations of them. The mystery looked intractable – until a discovery, published this year, raised hopes of solving the FRB puzzle. In January an international team of astronomers announced they had used radio telescopes across the globe – from Hawaii to the Netherlands – to pinpoint, for the first time, the precise position of a fast radio burst, which they labelled FRB 121102.

“We now know that this particular burst comes from a dwarf galaxy more than 3bn light years from Earth,” said Shami Chatterjee, of Cornell University, who was one of the astronomers involved in the project. “That simple fact is a huge advance in our understanding of these events.”

The fact that this burst was so incredibly distant but still detectable confirms Lorimer’s original suspicion that FRBs are produced by events that are very powerful. The burst from FRB 121102 was emitted at a time when life was first evolving on our planet. Yet it was still detectable on Earth 3bn years later, indicating it must have been caused by some of the universe’s most energetic phenomena. Scientists calculate this eruption emits, just for a millisecond, as much power as 500m suns

“We are the first to show that this is a cosmological phenomenon,” said Dr Casey Law, of the University of California, Berkeley, one of the project’s leaders. “It’s not something in our backyard. And we are the first to see where this thing is happening. Now our objective is to figure out why that happens.”

Since the first observation of a signal coming from FRB 121102, astronomers have seen a further eight bursts from it over the course of six months of study, a period that involved a total of 83 hours of observation. “The frequency of these bursts tells us that we are not witnessing some rare catastrophic event that heralds the destruction of a star or a black hole in deep space,” said Britain’s Astronomer Royal, Martin Rees. “These bursts are too frequent for that. Instead, we have to postulate some kind of regular phenomenon which is generating FRBs.”

The repeats of FRB 121102 are therefore of critical importance – though they still pose puzzles. It took 50 hours of observation before astronomers saw a repeat of its first signal, but later observed two that were only 23 seconds apart. The question for astronomers, then, is straightforward: what was recharging the vast batteries that were powering these incredibly energetic events and why are their repeats so erratic?

Several candidates had already been put forward by scientists to explain FRBs and some were simply thrown out of the window by the discovery of FRB 121102. For example, some cosmologists had suggested that stars flaring and erupting in our own galaxy might be responsible. The discovery that FRB 121102 was extremely remote and associated with a distant galaxy appears to kill that idea.

Then there was the proposal that FRBs could be caused by gigantic energy plants that were powering alien spaceships on intergalactic odysseys, a notion suggested by Harvard astronomers Avi Loeb and Manasvi Lingam. They speculate that advanced alien civilisations could have created huge transmitters to generate powerful bursts of electromagnetic energy that would push spacecraft fitted with sails to speeds approaching that of light.

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Crucially, observers on Earth would see a brief flash as the sail and the transmitter on its host planet swept rapidly across the sky, an extraordinary idea that the two astronomers admit is speculative. “Science isn’t a matter of belief, it’s a matter of evidence,” says Loeb. “It’s worth putting ideas out there and letting the data be the judge.”

Most astronomers are more orthodox in their approach to the problem, however. “You cannot say the probability of this idea is zero but I would say it is very close to that,” says Rees – a cosmologist based at the Institute for Astronomy at Cambridge. Instead, he thinks a class of stars known as magnetars are more likely to be sources of FRBs.

“Magnetars are a class of neutron stars with the special feature that their magnetic fields are far stronger even than the fields in pulsars – 100 trillion times more powerful than the one we have here on Earth,” adds Rees.

Conditions near a magnetar are extreme, to say the least. The heat they produce would vaporise an astronaut while their bodies would be torn apart by the colossal tidal effect of its huge gravity field. For good measure atoms themselves could not survive the intensity of the magnetic fields that magnetars radiate, says Rees. “The physics that you get near these objects is very exotic and you have got to understand to that if you are understand how fast radio bursts occur.”

What is clear is that the lines of the magnetic field around a magnetar could produce the effects we see on Earth. Astronomers speculate that these lines flail around the star and within the cloud of debris left over from the supernova explosion that gave birth to it. This process pumps energy into the surrounding gas and dust cloud which absorbs the energy and then occasionally releases it in abrupt, titanic bursts of energy into space.

The magnetar hypothesis is currently the most popular explanation for FRBs among astronomers. However, it still does not answer all the questions that are raised by FRBs. For example, the fact that only one FRB has so far been seen to repeat itself – while all the others are single events – is baffling and some astronomers suspect two different classes of objects might be responsible for these differences in behaviour. In other words, there could one source for FRBs that repeat and a different one for those that produce one-off bursts.

“The two types could be related – or they could be quite different. We just don’t know,” says Professor Mike Garrett, director of the UK’s Jodrell Bank Centre for Astrophysics. “The only fast radio burst that repeats itself is the only one whose location we know. So we need to pinpoint the others, which apparently don’t repeat, to see if they are also associated with distant galaxies.”

To achieve that goal will require a major boost in our study of these events, however. As Garrett explains, a single radio telescope can reveal the general direction of a burst but not with sufficient accuracy to allow an astronomer to say what galaxy it came from.

“To do that, we will need to use a combination of three or four radio telescopes – separated by at least 50 kilometres – to triangulate an observed burst and pinpoint its precise position.”

To achieve this goal, Garrett and his colleagues recently presented plans to the UK Science and Technology Facilities Council that would allow them to extend Britain’s e-Merlin radio telescope system, which links half a dozen instruments based round the country. This £15m upgrade would add new devices, including the Goonhilly Downs satellite station in Cornwall, to the network and so provide the necessary precision to allow astronomers to locate FRBs with unprecedented accuracy. But a decision on whether to approve the plan is not expected until the end of the year.

“If we get approval, we will be able to pinpoint fast radio bursts not just to specific galaxies but to particular parts of a galaxy,” added Garrett. “Are they found in the centres of galaxies or in the outer parts of galaxies? If it is the former, then this would suggest they are associated with supermassive black holes, which we find at the cores of galaxies.

Or if it is the latter, then that would suggest they are associated with newly formed stars which you find in the outer layers of a galaxy. However, until we carry out this kind of work, we are not going to solve the mystery of fast radio bursts. They are still an enigma for the moment.”

This point is backed by Rees. “What is really interesting about fast radio bursts is that they show there are still lots of things to be discovered when we explore the universe,” adds Rees. “And in this case, these objects – which involve objects releasing extraordinary amounts of energy – allow us to test physical laws under more extreme conditions than we could ever achieve in a laboratory. We can test the laws of physics to breaking point and perhaps discover new laws in the process. They are telling us something about the cosmos that we could not achieve through experiments carried out on Earth.”

Other energetic arrivals from space

Pulsars Discovered in 1967, pulsars are the relics of massive stars that have erupted as supernovae. These explosions leave behind tiny, dense objects called neutron stars and some rotate very quickly, emitting focused beams of electromagnetic radiation. To those in the beam’s path, the star appears as a regular pulse of radiation.

Gamma-ray bursts These are extremely energetic explosions that occur in distant galaxies and can last from a few seconds to several hours. Evidence from recently launched satellite observatories indicate that these gamma-ray eruptions come from the collapse of giant stars into black holes or neutron stars.

Gravitational waves Predicted by Albert Einstein in 1916 but not detected until 2015, these are ripples in the fabric of space-time and are triggered by massive events, such as the merger of two neutron stars that crash into each other.