Gamma-ray bursts – or death stars – are the most violent events in the Universe. They’re also shrouded in mystery. Robert Matthews investigates.
One April morning in 2009, an orbiting satellite picked up a 10-second burst of radiation coming from deep space. In seconds, the satellite had turned its ultra-sensitive telescopes towards the source – and just caught a flash of X-rays from the mystery object. Then it was gone.
Within half an hour, astronomers were using some of the world’s most powerful telescopes to look for the source. When they found it, they realised they were looking at something truly astonishing: the glowing remnants of a colossal explosion on the very edge of the Universe – so far away that the radiation had taken 13 billion years to reach us. That meant it could have only one possible source. The radiation had to have come from the single most violent event in the Universe: the detonation of a ‘death star’ or gamma-ray burst (GRB).
Astronomers later estimated that in that 10-second blast, the object they later code-named GRB 090423 had unleashed more energy than our Sun will emit during its entire lifetime. Every form of radiation, from gamma-rays and X-rays to heat and light, had burst from its core, while debris travelling at 99.99 per cent of the speed of light had smashed into the surrounding space. Even the very fabric of space and time was shaken by the explosion, rippling under its force.
Once the cosmic pyrotechnics were over, GRB 090423 was no more. But astronomers believe there is still something there – something at least as awe-inspiring as the events that spawned it: a gigantic black hole.
Big events trigger big interest
Even astrophysicists well-used to dealing with dramatic cosmic events admit to finding the ‘death star’ phenomenon mind-boggling. “Such extreme events of energy, distance, gravity and speed stretch the intuition of scientists – and the imagination of everyone,” says GRB expert Dr Brian Metzger of Princeton University.
You don’t need much imagination to hope that cosmic monsters like GRBs died out long ago. But, however comforting the thought is, it may not be the case. GRBs can and do still happen, and not only at the edge of the cosmos. Astronomers have identified potential candidates for future GRBs on our cosmic doorstep. Some even claim that Earth itself was once blasted by a GRB, with consequences as devastating as those of a global nuclear war.
Ironically, it was attempts to prevent just such a war that first revealed their existence. During the 1960s, at the height of the Cold War, the US Department of Defense set up a network of satellites codenamed Vela, fitted with radiation detectors to spot any attempt by the Soviets to test nuclear weapons in space. In July 1967, the satellites seemed to confirm the United States’ worst fears, detecting flashes of gamma radiation out in space. However, more careful analysis quickly revealed that the flashes were unlike those expected from a nuclear weapon. Unable to say what they’d detected, Pentagon advisors declassified the data in 1973, allowing civilian scientists to suggest explanations.
It proved far from simple. The problem was all in their name: a burst of gamma-rays would appear somewhere in space, but vanish before astronomers could get a telescope to examine the source. Early gamma-ray satellites couldn’t pin down their position accurately, either. It wasn’t even clear if the bursts were relatively minor events close to the Solar System, or colossal explosions on the other side of the Universe.
The breakthrough came in May 1997, when an orbiting satellite detected a GRB and established its location fast enough for ground-based telescopes to study it and, for the first time, measure its distance. The result was astonishing: GRB 970508 turned out to be three billion lightyears away, yet was still visible from Earth. And that meant that in its 15-second outburst, this death star had unleashed more energy than the annual output of a whole galaxy.
For astrophysicists like Dr Metzger, this immediately ruled out all but one explanation for GRB 970508. It could only be the result of the collapse of a colossal star into an incredibly dense object – possibly even a black hole. With a mass at least 20 times that of the Sun, such giant stars burn brilliantly and rapidly through their hydrogen ‘fuel’. Then they start to collapse under their own gravity, driving temperatures and densities ever higher until their whole mass is packed into a region barely 100km across, forming a so-called neutron star, or even a black hole.
Recipe for a burst
“By studying such GRBs we get a front-row view of them in their crucial formative phase,” says Dr Metzger. “We’re confident that some combination of these objects is ultimately responsible for powering these GRBs – though it gets trickier when one tries to distinguish between a voraciously feeding black hole or a rapidly spinning neutron star with a strong magnetic field.”
Such questions are now being tackled using data sent by purpose-built satellites like Swift which are detecting new GRBs at the rate of around one a day. But these are adding new mysteries of their own.
One of the most baffling centres on so-called short GRBs, which last barely a second or so. Their fleeting appearance stymied attempts to study them in any kind of detail until 2005, when Swift detected the afterglow of one in a galaxy 800 million lightyears away. Although only a few dozen have since been identified, the evidence suggests they’re the result of a cataclysm even more dramatic than the collapse of a giant star. “The favoured idea is that they are produced when two compact objects, such as two neutron stars, coalesce,” says astrophysicist Professor Nial Tanvir of the University of Leicester. “It does fit in pretty well with the observations we have, but I think it’s quite possible we’ll find that some short bursts are produced by different means.”
More bursts, more questions
And then there are the GRBs that refuse to fit into any neat category – such as GRB060614, discovered by Swift in a galaxy over a billion lightyears away. Its blast lasted for over 100 seconds, putting it firmly in the long GRB camp, and thus linking it to the collapse of a giant star. Yet when the world’s most powerful telescopes looked for remnants of the original star, they found nothing. Had everything been engulfed by the final black hole or had something entirely different caused the burst? “The most intriguing and unexplained mystery is the vast complexity and variety of the fading behaviour of GRBs,” says Professor Carole Mundell of Liverpool John Moores University. “Sudden fades, slow fades, sudden re-brightenings… We need to look harder to find the origin of the observed behaviour.”
Understanding the origin and behaviour of GRBs is one of the greatest challenges facing astrophysicists. But some believe the results aren’t just of academic interest. They think GRBs have threatened the very survival of life on Earth in the past and may do so again. In 2003, a team of astronomers led by Professor Adrian Melott at the University of Kansas announced that a GRB in our own galaxy may have been responsible for the so-called Ordovician Extinction, during which most life on Earth perished around 450 million years ago. Could we face a similar fate today? Astronomers have long known that super-massive stars capable of spawning GRBs still exist in our galaxy. Known as Wolf-Rayet stars, these could collapse to black holes at the end of their lives. And if one exists even several thousand lightyears away, any resulting gamma-ray blast would pose a threat to life on Earth.
Worryingly, astronomers know of several Wolf-Rayet stars close enough to put our planet in the kill zone. Most attention has focused on a star around 8000 lightyears away, known as WR104. It’s certainly massive enough to turn into a black hole at the end of its life and calculations by Prof Melott and his colleagues suggest it’s close enough to affect the Earth. “But there’s a caveat,” cautions team member Dr Brian Thomas of Washburn University, Kansas. “The rotation axis must be pointed directly at us, as the emission from a GRB is strongly beamed – and we must fall in that beam-line in order to receive the full effect.” The good news is that the axis of WR104 is around 30-40° off, which should protect us from the worst effects. And that’s presuming it even turns into a GRB, which is far from certain, says Prof Tanvir. “One of the things we’ve already learned is that GRBs seem to be more common in galaxies that are chemically more primitive than ours.”
Add in the fact that calculations suggest that just a few GRBs may take place in our entire galaxy every billion years and it seems there’s little immediate cause for concern.
Yet even after almost 40 years of study, some very basic questions about GRBs remain unanswered. Exactly how do they form? How many different types are there? And what lurks in their place after they have exploded?
“We may need to go back to the blackboard and consider entirely different models,” says Prof Metzger. “But the relevance of GRBs to humans is that they provide an almost daily reminder of the sheer power and majesty that the Universe can create.”
Robert is a science writer and visiting professor of science at Aston University.