Armagh Observatory: School Student Essays
Gamma-ray Bursts
Debbie Bond, Dalriada school, Ballymoney, 30 March 2001
Gamma-ray bursts (GRBs) are the most powerful explosions in the universe. The explosions release most of their energy as gamma-rays, hence the name. They occur randomly in time at any location in the sky and are extremely diverse. The shortest yet seen lasted only 5 milliseconds, the longest a few hours, though most of them last between 1 and 100 seconds.
Gamma-ray bursts were first detected in the 1960s. American spy satellites were on the look out for illicit nuclear tests, but what they saw instead were mysterious flashes of gamma-rays from space.
So how do you study something that goes off unexpectedly, releases more energy than the Sun will in its entire lifetime, and vanishes again within seconds? The trick is to study the afterglow emitted by the rapidly cooling fireball. Afterglows can be studied for days or even weeks. The problem was detecting accurately where they are and quickly. BATSE can spot a burst and pass the information rapidly to telescopes round the world. But unfortunately it is not very good at pinpointing the exact location of the burst. Its best guess is within 2 degrees of the sky, or four times the diameter of the full Moon. The more accurate but slower way was to use an interplanetary network of spacecraft. Comparing the time that a GRB arrives at the different spacecraft gives a very accurate position. However, this could take as long as a month. The solution was BeppoSAX, which was launched in 1996. It can pinpoint GRBs with great accuracy and quickly. This allowed astronomers to see the afterglow.
One question was where were the gamma-ray bursts coming from. In 1992 results were announced from BATSE. It showed that all the bursts were randomly distributed over the sky with no bias to the galactic plane or centre. Therefore we must be in the centre of a uniform sphere of GRB sources. The explanation was that they stretched out to the farthest reaches of the universe. This showed that they possibly are cosmic not galactic. The outer edge is where the expanding universe has redshifted the GRBs out of the energy bands that the satellites can see.
After one GRB, on the 28 February 1997, a faint fuzz of light was found surrounding the point of light left behind. Could it be the glow from a faint, distant galaxy? The Hubble team looked and saw no signs of it fading. This is what you would expect if the light was from a galaxy. But the Keck team said the fuzz had faded below their detection limit. The Hubble team reported that the fuzz was not moving, suggesting that it was very distant. But Patrizia Caraveo and several collaborators in Europe and the US reported that they had reanalysed the Hubble data and found that it was moving so quickly it had to be within our galaxy. Rees and Ralph Wijers issued a paper pointing out that the afterglow had hung around too long for the GRB to be galactic. If it were local, the fireball would have less energy and slow down more quickly. But it was still visible a month after it was first detected.
The next burst that BeppoSAX spotted was on the 8 May 1997. This time the Caltech team managed to get a spectrum of the object. From this they found that some of the light was being absorbed. They calculated that the intervening object was at least 7 billion light years away. This would mean that it would have to be cosmological.
More evidence for the bursts being cosmological came after a GRB on the 10 May 1999. Galama and Vreeswijk managed to take the objects faint spectrum. This provided them with little information but it would allow its distance from the Earth to be calculated. It implied a distance of some 9 billion light years.
Using this along with the observed brightness of the burst they would be able to calculate the energy of the explosion. But a problem arose, if it was assumed that the energy of the explosion was emitted equally in all directions, the figure of 1.4 X 1053 ergs was obtained. This is as much energy as if the Sun converted a tenth of its mass instantaneously into radiation, but there is no known way that could happen. Instead they assumed that GRBs emitted their radiation in two opposite beams or jets. This would solve the problem, as the energy required would be much less.
Fortunately, the new observations supported this. It was observed that a few days after the burst, the afterglow suddenly starts to fade more rapidly than before. This could not happen if the emitting fireball was a sphere. At first the fireball is expanding at nearly the speed of light, and according to the theory of relativity this means we can only see the small part of the sphere that is moving directly towards us. As the fireball cools, it becomes dimmer, but also slows down, and because of the lower speed more of the sphere can be seen. So the apparent brightness falls only slowly. If the radiation is as a jet then the same relativistic effects are at play. But after a while, the expansion speed is low enough for us to see the entire jet. Further cooling and slowing doesn’t bring more emitting matter into view and the apparent brightness starts to decrease much more rapidly. Therefore, it is unlikely that it is a sphere. Also models predict that long narrow jets of material could cool this quickly, but not a spherical hot region.
This evidence points to the energy being emitted as rays but there is still a major question that is how are they formed. There are many theories but it will be difficult to prove which one or possibly ones are true. Astronomers agree that GRBs occur when matter heated to billions of degrees explodes at close to the speed of light. "Shocks" in the fireball generate gamma-rays. But nobody has come up with a completely convincing power source for the fireball.
Stan Woosley believes that bursts happen when a very massive star suddenly turns into a black hole. According to computer simulations this "collapsar" should make beamed explosions. When the core of a supermassive star collapses into a black hole, the surrounding material would fall or "accrete" onto the black hole, releasing vast amounts of energy. Accretion of matter into a black hole at tremendous rates, about 1 million Earth masses per second, could generate a jet. Galama’s discovery in April 1998 fits nicely into Woosley’s collapsar model. He saw an unusual supernova coincided with a run-of-the-mill gamma-ray burst. A nearby collapsar should look like a supernova with a weird spectrum. But the resulting beams of radiation seem not to have been directed exactly towards Earth; otherwise the accompanying gamma-ray burst would have been much brighter.
Another popular theory could also produce beaming. A binary pair of neutron stars will gradually spiral together, losing their orbital energy as they emit gravitational waves. Just before they finally merge into a black hole, they should form a flattened, rotating spheroid, allowing jets of ultrahot matter to escape at the poles. Because neutron stars are so small and compact, these collisions only last for a few seconds. This is just right for making GRBs. There are plenty of star systems containing two neutron stars orbiting each other and every galaxy has several thousand of them.
However, there are problems with this model. The biggest is that when two neutron stars collide they form a black hole, which could swallow up all the material from both stars and allow nothing to escape, not even radiation. However, it is possible that some of the debris from the explosion is not swallowed up by the black hole but remains trapped in orbit around it. The orbiting material would be spinning very rapidly in a flat disc, and the energy from this spinning could cause the GRB. The spinning material would be threaded through with a magnetic field left over from the neutron stars. But because different parts of the disc would be spinning at different speeds, the field lines would become twisted. This would quickly become unstable and release a sudden burst of energy. Woosley says, that this scenario is not limited to colliding neutron stars. Anything that produces energetic material spinning round a black hole would do the trick. A collapsar could produce GRBs in the same way but with much greater energy.
Another problem with this model is that there is a hint that several bursts were coming from the same part of the sky. But if two neutron stars collided that would definitely be a one-off. Woosley imagines a scenario in which a black hole slowly captures another star. On the first pass, the black hole could break the star into pieces. On the second pass, the pieces could be drawn inwards. One by one, the pieces would form discs around the black hole. The magnetic field lines within the disc would then twist and each disc would release its energy in a GRB, just as for the colliding neutron star model. This way there would be a series of bursts from the same source.
Another possible reason is that gravitational lensing by wormholes might explain some bursts. Wormholes would have negative mass, and exert a repulsive gravitational force. They should therefore deflect light away from them to converge on a bright curve called a caustic. So as a wormhole moved across the line of sight to an active galaxy, it would drag the caustic along with it, producing two bursts – an anti-FRED followed by a FRED, a couple of years apart. A "FRED" (Fast Rise, Exponential Decay) rapidly reaches a peak of intensity then gradually fades away. An anti-FRED peaks slowly then disappears fast. Wormhole with a negative mass about a tenth of the Sun’s mass could explain up to 5 per cent of gamma-ray bursts.
We know GRBs exist we are still not sure how or where they form. Another question is what effect do they have. Stephen Thorsett of Princeton University has been thinking about the implications for life on Earth should a nearby object in space suddenly emit a powerful burst of gamma-rays. The radiation would strip away the ozone layer and cause mass extinctions. If the bursts are distant and immensely powerful, similar events probably happen in our galaxy every few hundred thousand years. Most of these would not cause too much damage. A gamma-ray burst at the galaxy’s centre, 27 000 light years away, would disturb the ozone layer no more than did the 1991 eruption of Mount Pinatubo in the Philippines. But if a burst originated within a couple of thousand light years of the Earth, it would expose the atmosphere to as many gamma-rays as the detonation of all the planet’s nuclear weapons. The ozone layer would disappear for several years, allowing deadly ultraviolet radiation to reach the Earth’s surface and kill many species. Thorsett estimates that these lethal gamma-ray bursts should occur once every few hundred million years. The gamma-rays would produce airborne radioactive isotopes, such as carbon-14, which would fall to the ground. Geologists will probably not be able to detect signs of a past burst, as the isotopes would have completely decayed. There are potential sources of gamma-ray bursts relatively close to the Earth.
Brian McBreen and Lorraine Hanlon of University College Dublin suspect that the formation of the solar system was hurried along by a nearby gamma-ray burst. Rather than aborting the birth of planets, the flood of energy may have melted primordial dust grains, seeded the formation of meteorites and helped form the rocky planets. They suggest that all the chondrules (rocky beads rich in iron and silicon minerals) in the Solar System formed in a matter of minutes 4.5 billion years ago, when a gamma-ray burst seared the dust and gas circling the Sun with intense X-rays and gamma-rays. They calculate that a gamma-ray burst within 300 light years would have flooded the dusty disc circling the young Sun with enough energy to fuse up to 100 Earth masses of material into droplets that cooled into chondrules. These, and the dust from which they are formed, are rich in iron, which would have soaked up X-rays and gamma-rays very efficiently. They also think that the dense chondrules settled quickly into the plane of a protoplanetary disc and speed the formation of planets. Their theory implies that solar systems such as ours are rare.
How they are formed, where they are formed and the possible effects are still not definitely know. Although there are many possible reasons some have more evidence supporting them than others. The most likely reasons are that they are formed at the edge of the universe, are formed by either a very massive star collapsing or two neutron stars colliding, and the most likely effect is mass extinction of most life on any planets nearby. In the past a lot of the theories have be discounted after new information showed that they could possibly be true. It may also turn out that none of the current theories are true. But who knows when the discovery of the truth will be made. It may not be all that long due to the constant improvements in technology which are giving us a better view of space.