They might flare brighter than all the stars in the universe combined, but gamma-ray bursts can prove mysteriously difficult to find, writes Yvonne Cunnigham.
When it comes to fireworks, it is difficult to top a stellar explosion known as a gamma-ray burst. When these stars explode, for a few seconds they give off more light than all the stars in the universe combined.
Now Irish astronomers have helped take an important measurement associated with gamma-ray bursts, the speed of the material ejected when these stars go off.
Gamma-ray bursts are formed when a very massive star collapses into a black hole. "When this happens the exterior is ejected into space in two jets of material in opposite directions," explains Evert Meurs, director of Dunsink Observatory and leader of the Irish team involved in this project.
The Irish contribution to the project was made by Susanna Vergani, a Dublin City University PhD student. From Dublin, she was able to remotely operate the robotic Rapid Eye Mount (Rem) Telescope at the European Southern Observatory in La Silla, Chile.
Gamma-ray bursts can't be seen by the naked eye, but they generate an afterglow which is visible. It is this afterglow which the telescope in Chile observed.
"Susanna Vergani operates the telescope over the internet, she gets messages sent to her mobile phone from it," says Meurs.
Vergani analysed the data that came from a gamma-ray burst.
"When these particular gamma-ray bursts were observed she was engaged with the data reduction to apply certain corrections to the measurements of brightness, and having done that she could get a picture of how the brightness changes with time," says Meurs.
Vergani then compared the brightness she measured with models of what these explosions should look like.
"She has been interpreting and comparing the values with predictions that are taken from the literature of accepted models of gamma-ray bursts. She has worked it out so that we can draw conclusions about the speed of the material going out from the gamma-ray bursts," says Meurs.
Nobody knows for certain why these explosions occur. One possible explanation is that when a star collapses in on itself to form a black hole, it causes the material on the exterior of the star to be released in jets.
"A star collapses at the end of its life into what is called a black hole and in the course of that collapse it also has an effect on the matter immediately surrounding it causing gamma-ray bursts. The exterior is ejected into space in two oppositely directed jets of material, in a very narrow column at exceptional speed," says Meurs.
"What is exploding and heading to earth is probably some plasma which is matter at a very high temperature so that it disintegrates, and this is what causes the radiation."
Calculating the speed of a jet is complicated. "There are models of these kinds of explosions where matter is released in jets. These models show expected changes in brightness related to speed of ejection. So, by comparing our values with the values in the models we can calculate the speed of ejection," he says.
The value they have found for the speed of ejection is close to the speed of light. "We have a figure for the speed of ejection that is 99.997 per cent of the speed of light," says Meurs.
Not all gamma-ray bursts are observable. "Only about 50 per cent of exploding stars have this afterglow, the reason is still a mystery at the moment," says Meurs.
There are a couple of possibilities. "gamma-ray bursts go off in surroundings of gas and dust, and optical light can get blocked by dust, simply by the dust obscuring the light," says Meurs.
"The second possibility is that we are looking at gamma-ray bursts that are very far away and they are simply so dim that we can't see them," he adds.
Observations of the afterglow can tell us about the type of star that exploded. "We can tune in to the afterglow and do very detailed investigations looking at the spectral lines. These are lines in the optical spectrum which can tell you about the physical characteristics of the exploding elements," says Meurs.
There are no direct practical applications for this research, but sometimes there are unexpected spin-offs.
"Many objects with very high energy have their X-rays observed by telescopes, the technology developed to do this is now used in X-ray scanners at airports," says Meur.
In Ireland the research is mainly funded by Science Foundation Ireland.