Gamma-Ray Bursts are very short and very strong bursts of gamma rays from unknown sources. They come from every direction in space and are seen about once every day. They were discovered in the 1960s when the USA launched a series of “Vela” satellites to monitor nuclear tests by the USSR. They were designed to look for bursts of gamma rays on the Earth, but soon discovered that the bursts they had recorded were coming from outer space.
GRBs have very high energies but only last for between 0.01 and 1000 seconds. They can last for any length of time within this range but they are usually either about 0.2s long or 20s long. This suggests that there may be two different types of GRB, caused by different things.
1400 GRBs have been recorded so far, but only 3 have ever been seen to repeat. The short duration of the bursts makes it very difficult to pinpoint exactly where they came from. It is usually only possible to narrow this down to an area of sky slightly bigger than the Moon, which may contain thousands of stars and galaxies. When a GRB is recorded, telescopes are quickly moved to look at the area to try and find an object that may have caused the burst. So far, out of 1400 GRBs, only two of them have been connected to likely sources.
The three repeating sources may give some clue as they seem to be in the same area as supernova remnants. This suggests that repeating GRBs may be connected with neutron stars, which are found at the centre of many supernovae.
One way of learning more about the bursts is to look at the pattern of light coming from them, called their spectrum. The spectra of repeating and non-repeating GRBs are a different shape, which suggests that they are caused by different things.
The fact that the GRB sources can’t be found means that the distances to them are unknown. This makes it very difficult to measure their energies. The normal way of measuring the energy given out by a distant source is to look at the amount of radiation reaching the Earth, then use the distance to the object to work out how much energy must have been given out in the first place. If the distances are unknown, this is impossible. From the energies that we see, the actual energy given out by the GRB could differ by as much as 100,000,000,000,000,000,000 times, depending on whether the source is within our own galaxy or outside it.
The very high energies calculated for GRBs suggest that they are produced either by the collapse of a white dwarf to form a neutron star, the collision of two neutron stars or the collision of a neutron star and a black hole. The black hole theory seems to be the most likely at the moment.
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Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001