Posted: October 15, 2008
ESA’s Integral gamma ray observatory has detected several faint gamma-ray bursts, confirming the existence of an entirely new population of weak bursts.
Gamma-ray bursts (GRBs), powerful explosions of energy triggered by the collision of neutron stars or black holes, or by supernovae, are usually the brightest objects in the gamma ray sky, albeit for a very brief time. They are thought to occur as often as 1,400 times per year, but because no one knows when and where they will occur, most of them are missed. During four and a half years of operation Integral has detected 47 GRBs, and with the most sensitive detector ever launched to space, it has revealed the signatures of the faintest GRBs and their weak X-ray and visible afterglows.
iGamma-ray bursts (GRBs) are the most powerful explosions in the Universe, but Integral has detected a new population of faint GRBs located in our cosmic neighbourhood. Image: ESA, illustration by ESA/ECF.
At first, astronomers working on the IBIS data thought that since GRBs are hugely powerful explosions, one which is perceived as faint will be located far away. But the GRBs detected by Integral appeared to be situated in our own cosmic backyard, within the nearby clusters of galaxies.
"If the bursts we have studied are so 'close' in cosmological terms, it means that they are faint from the beginning," says Professor Lorraine Hanlon from the School of Physics, University College Dublin. "From this we can deduce that the processes triggering them could be less energetic than those generating the more powerful bursts we are more used to observing."
Some of the possibilities brought to the table include the collapse of a star, but one that does not show the characteristics of a typical supernova, the merger of two white dwarfs, or the merger of a white dwarf with a neutron star or a black hole.
"Past observations had already hinted the existence of faint GRBs, and thanks to Integral's sensitivity we can now say that an entire population of them exist," says Hanlon. "Actually, their rate may even be higher than that of the most luminous GRBs but, just because they are weaker, we may be only able to see those which are relatively close by."
With Integral only half way through its mission timeline, many more observations will certainly help astronomers to explore the nature of this newly observed population and to understand the mechanisms that power faint GRBs.