Gamma-Ray Bursts are short bursts of very intense gamma rays from unknown sources. They come from every direction in the sky and are seen about once every day. They were discovered in the 1960s when the USA launched the 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 10-2 and 103 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 mechanisms.
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 objects. 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 the 1400 GRBs, plausible sources have only been identified for two of them.
The three repeating sources may give some clue as they coincide with the positions of supernova remnants, suggesting that repeating GRBs may be associated with the neutron stars that are found at the centre of many of them.
One way of attempting to analyse the bursts is to look at the pattern of light coming from them, called a spectrum. There is a big difference between the spectra of repeating and non-repeating GRBs. Non-repeating GRBs have many peaks in their spectra, which are very complex, while repeating GRBs only have a single peak. This suggests that they are caused by different things.
One consequence of being unable to find the GRB sources is 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 determine how much energy must have been given out in the first place.
The energy is measured in total energy observed per cm2 of the detector, and is called the Fluence, S. S can range from 10-7 erg s-1 to 10-3 erg s-1. Without the distance, these observed energies mean that the actual energy given out by the GRB can differ by 20 orders of magnitude depending on whether the source is within our own galaxy or outside it. Their isotropic distribution suggests, however, that they are non-local as if they originated within our own galaxy there would be a concentration of GRBs from the disc region of the Milky Way.
The very high energies calculated for GRBs suggest that they originate in very energetic events such as 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 latter currently seems to be the most likely.
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Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001