SOME fearsome cosmic engine is firing missiles at us. Travelling a shade slower than light, this packs a terrific punch. Luckily for us, these missiles are tiny subatomic particles. A bullet travelling at the same speed would carry so much energy that it would be the end of humanity if it struck Earth.
Physicists have struggled for decades to explain the origin of these high-energy particles, which they see hitting our atmosphere. While there are assorted theories to explain where cosmic rays come from, none has previously been able to account for the extraordinary range of energies they have. But now Charles Dermer of the US Naval Research Laboratory in Washington DC has pulled all the different strands together and an all-encompassing theory of cosmic rays is starting to emerge. His calculations show that they could all come from exploding stars.
Most cosmic rays are ordinary protons and ions, which have energies of billions to trillions of electronvolts (eV) when they hit the Earth’s atmosphere. But a few are far more energetic than this. They can have energies of more than 1020 eV – 100 million times more energetic than the particles whizzing round today’s most powerful particle accelerator. What natural phenomenon could be responsible for producing particles with such stupendously high energies?
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Lower-energy cosmic rays, with between 109 and 1012 eV, are probably created when giant stars end their lives in supernova explosions. When such a giant runs out of the nuclear fuel that keeps it burning, it collapses to form a neutron star and blows off a shell of plasma, causing shock waves that can probably accelerate protons up to as much as 1014 eV. But particles of higher energy are far more difficult to account for.
There are several theories for their origin, including active galaxies powered by supermassive black holes, and hypothetical ultra-heavy particles, but they all have their problems. Dermer has instead built on an idea conceived in 1995 by Eli Waxman of the Weizmann Institute in Rehovot, Israel, and others. He believes all higher-energy cosmic rays come from the supernova’s even more violent cousin: the gamma ray burst (GRB).
GRBs reveal themselves in brilliant flashes, of high-energy photons called gamma rays lasting from seconds to minutes. These bursts probably mark the deaths of still more massive stars. Theories vary about what exactly happens: the core may implode directly to become a black hole, or it may pause briefly, forming a rapidly spinning neutron star before collapsing again. But the result is much the same. All the gravitational energy spat out by the forming black hole is concentrated into nature’s most ferocious blast wave, travelling outwards at 99.999 per cent of the speed of light. The blast is driven by shock waves spreading through the hot plasma surrounding the burst. Threading through the plasma are magnetic fields that, together with the shocks, whip electrons in the plasma up to high energies. These electrons are thought to spiral around the magnetic field, emitting electromagnetic waves that are then Doppler shifted to the ultra-short wavelengths of gamma radiation.
Waxman’s idea was that these shocks could also accelerate cosmic rays. Imagine, for example, an ordinary proton left over from the stellar wind that once blew from the original star. As the blast wave passes, it gets caught up and starts bouncing around, deflected by kinks in the magnetic field. It travels repeatedly from one side of the shock wave to the other. With each deflection, the proton picks up more energy, like a rubber ball bouncing between two converging walls. This happens thousands of times until it gains enough energy to break through the shock wave and escape.
According to Waxman’s latest calculations, shortly to be published in The Astrophysical Journal, the magnetic field in the blast wave has just the right strength to accelerate cosmic rays to ultra-high energies. If it were much stronger, it would bend a proton on a tight corkscrew path, causing the proton to radiate electromagnetic waves that carry away its energy. If it were too weak, the magnetic field would be unable to hold a proton long enough for it to reach these high energies.
Instead the field can keep our proton long enough for its energy to climb as high as 1020 eV. At this point, Waxman calculates, the proton will escape the clutches of a GRB. It flies away from the burst, straight out of the galaxy that held it and into intergalactic space. There it spends years travelling at a shade less than the speed of light. The weak magnetic fields that thread intergalactic space bend its path only slightly. There is a chance that the proton will eventually enter our galaxy and hit Earth.
When it smashes into the nucleus of an atom in the upper atmosphere, it will create a tiny fireball that in turn sparks off a shower of high-energy particles. By the time the shower reaches ground level, that original cosmic ray might have spawned a million offspring.
One anecdotal proton is not enough, of course. And Waxman’s model explains only cosmic rays with the very highest energies. Dermer’s aim is to explain the whole cosmic ray spectrum, which stretches over a huge range from 1011 to 1020 eV (see Graph). To fit this spectrum, Dermer had to add up all the cosmic rays coming from GRBs. He recruited Stuart Wick, also at the Naval Research Lab, and Armen Atoyan from the University of Montreal in Canada. From measurements made by NASA’s Compton Gamma Ray Observatory and other spacecraft, astronomers infer that about 1000 bursts go off in the universe every day, each one signalling the birth of a black hole. And Dermer thinks that each burst pumps a huge amount of energy into cosmic rays – many times as much as the sun will emit in its lifetime.
Cosmic scattering
The team built a computer model that follows these cosmic rays as they travel across the cosmos. In this model the particles are scattered by magnetic fields and have their energy sapped by collisions with the microwave background radiation left over from the big bang. Eventually some of them reach Earth, where, according to the model, they have a spectrum that fits very closely with experimental results.
At least, it fits above about 1017 eV. Below that energy, Dermer’s model shows incoming cosmic rays have a hard time of it: they get twisted around so much by intergalactic magnetic fields that most of them would take longer than the age of the universe to reach us, and most of the rest get blown back by a wind of plasma streaming from the Milky Way.
To explain all the cosmic rays with energies between those churned out by supernovae and those from distant GRBs, Dermer and his colleagues need something else: a burst inside our galaxy. Although GRBs are rare events, the team claims there is a fair chance that one would have gone off just a few thousand light years away within the past few hundred thousand years. And that’s good enough. We would still be inside a bubble of cosmic rays blown by that burst, and these would fill the gap in the spectrum, the group reports in a paper to be published in Astroparticle Physics.
So gamma ray bursts could account for high-energy cosmic rays, from 1014 eV all the way up to 1020 eV. “We are the first group to be able to fit all this data with one type of source,” he says. It is a tidy picture. All cosmic rays come from the deaths of giant stars – supernovae for low energies, GRBs for high.
However, not everyone accepts Dermer’s model. Waxman believes that although GRBs can account for the small number of ultra-high energy cosmic rays observed, the total energy coming from them is not enough to explain the much higher numbers of cosmic rays with intermediate energies.
Certainly, when the energy in gamma rays and all the afterglow radiation from a burst is added up, it does not come to anything like as much as Dermer needs for cosmic rays. Dermer does not think this is a problem, though. He believes the radiation we see from GRBs is only a fraction of their true power, and almost all the energy in the blast is pumped into protons and other ions.
One sign that he might be right comes from a particular burst seen in 1994 by the Compton Gamma Ray Observatory, named GRB 941017. At the time it seemed like a fairly ordinary burst, but in 2003 a team at the Los Alamos National Laboratory in New Mexico re-analysed the data, combining observations from two different instruments on board the observatory. They found that as GRB 941017 faded, it changed. For the first couple of minutes, the burst behaved as expected. Then it started sending out very high-energy gamma rays.
Most models struggle to explain the amount of gamma rays shot out at such energies. However, in a paper to be published in Astronomy & Astrophysics Letters, Dermer and Atoyan say their model can explain this. They think an intense blast of cosmic rays hit unfriendly fire a few minutes after leaving GRB 941017. A nearby gas cloud could have reflected some light from the burst back at the particles. Interactions between this reflected light and the cosmic rays would have been enough to destroy many of the protons and other particles. A by-product would be some outrageously energetic electrons, which would send out high-energy gamma rays. And so GRB 941017 could be telling us that GRBs really do squirt out huge quantities of cosmic rays.
Alan Watson at the University of Leeds in the UK is a cosmic ray experimentalist with no great regard for theoretical speculation, but he is positive about Dermer’s model. “He is one of the modellers I do pay some attention to, and I think the burst idea is pretty exciting,” Watson says. But, he adds, many rival ideas to explain ultra-high energy rays are not quite dead yet. Some astronomers argue that cosmic rays could come from distant objects called active galaxies, while others speculate that they are the product of unstable exotic particles called “wimpzillas” created in the big bang. But these ideas have their problems: most active galaxies are too far away to send the highest energy cosmic rays to Earth, while wimpzillas require speculative new physics.
Ultra-high energy cosmic rays are not the only part of the spectrum that is up for debate: there are also other ideas to explain cosmic rays with intermediate energies. It may be that in ordinary supernova remnants, cosmic rays inject energy into the magnetic fields, effectively strengthening their own prison walls. If so, they might remain trapped up to truly high energies. Other researchers have suggested that low-energy cosmic rays could be re-accelerated by shocks in star-forming regions, or by a giant shock wave that is thought to surround the galaxy. But Dermer believes these are unnecessarily complicated, jury-rigged models.
We should not have long to wait to find out if he is right. Three grand observatories are nearing completion, each designed to explore the high-energy universe. Perhaps the oddest is being built over the next five years at the South Pole. IceCube will point downwards, staring straight through the Earth with a compound eye made of 5000 bulbous photomultiplier tubes, looking for high-energy neutrinos that have slipped most of the way through the planet. Dermer and Atoyan predict huge numbers of neutrinos will accompany GRBs, so if IceCube sees strong flashes of neutrinos coinciding with bursts, their model will be vindicated.
Another test will come from GLAST, the Gamma Ray Large Area Space Telescope, due to launch in 2007. If Dermer is right, GLAST will see a lot of very energetic gamma rays above 108 eV coming from bursts.
Even if these telescopes sink Dermer’s comprehensive model, GRBs might still account for the most energetic cosmic rays. At the moment, the two most powerful observatories disagree even on what happens at the highest energies. The HiRes experiment in Utah looks at the upper atmosphere for fluorescent nitrogen excited by the original cosmic ray and its secondary particles. It sees only a few cosmic rays above 1020 eV, which could fit the GRB model. But the AGASA observatory in Japan, which detects charged particles hitting ground level, seems to see far more than GRBs could explain.
The controversy should be resolved by the giant Pierre Auger Observatory in Argentina. It will eventually watch 3000 square kilometres of the atmosphere, counting both fluorescent nitrogen flashes and cosmic rays on the ground. A quarter of the telescopes and detectors are working already, and by the end of this year the growing observatory may have gathered enough results to fill in the spectrum. If that does not kick out the GRB model, it will seem a good bet that those outrageously energetic cosmic rays really are fired at us by newborn black holes.
Cosmic destroyers
If cosmic rays do come from gamma-ray bursts, they could be a lot more dangerous than anyone had suspected. What if a burst went off only a few thousand light years away? GRBs are not spherical explosions, but erupt in two narrow cones like a double-ended firework. So 499 times out of 500 the main blast would be aimed away from Earth, and we would be all right. But if we were unlucky enough to be staring straight down the barrel, it could be fatal.
Adrian Melott of the University of Kansas in Lawrence, believes that 443 million years ago, a GRB may have killed almost all life on Earth (Â鶹´«Ă˝, 27 September 2003, p 17). In a controversial pre-print last September, he described how a GRB might account for the second largest mass extinction our planet has ever suffered, at the end of the Ordovician period.
The gamma rays would destroy the ozone layer, letting in ultraviolet light from the sun, and create nitrogen oxides to darken the skies with smog and turn the rain to acid. But if most of the energy is hidden in cosmic rays, things could get far worse. After the initial flash of gamma rays, there would be a sudden onslaught of neutrons, then a longer irradiation by protons and ions. This radioactive rain could shred the DNA of organisms on Earth, causing cancers and other mutations. Charles Dermer at the US Naval Research Laboratory in Washington DC is now calculating how the process would unfold, to find out just how big a dose of each kind of radiation would hit Earth, and how much damage it would cause.
We probably get hit only every few hundred million years, so don’t worry too much. But it is just possible that the first wave of gamma rays from a nearby burst is approaching the solar system as you read this. We won’t know until it hits.