Now a team of scientists, led by astronomers from the University of Leicester, believes they have found an answer to the puzzle.

Their research results, published in Nature on April 4, indicate that gamma ray bursts are caused by the death of a star so huge that when it dies in a supernova, its core collapses to form a black hole, resulting in an intense outburst of gamma rays.

Dr. Julian Osborne of the University of Leicester explained,"Until now it was unclear whether gamma ray bursts were caused by a supernova explosion of a giant star collapsing into a black hole, or by the coalescence of two neutron stars.

"Each event could result in an intense outburst of gamma rays, followed by an X-ray afterglow. By analyzing this X-ray afterglow we believe we have determined the gamma ray burst origin."

The scientists used the EPIC (European Photon Imaging Camera) instrument on the European Space Agency's (ESA) XMM-Newton space telescope to capture the X-ray afterglow of a recent gamma ray explosion in a galaxy 10 billion light years from Earth, and then conducted a detailed spectral analysis of the data.

The results were a great surprise.

Dr. James Reeves of the University of Leicester added, "For the first time ever, traces of light chemical elements were detected -- including magnesium, silicon, sulphur, argon, and calcium -- neutron star collisions are not expected to make these. Also, the hot cloud containing these elements is moving towards us at one tenth of the speed of light.

"This suggests that the gamma ray burst resulted from the collapse of the core of a giant star following a supernova explosion. This is the only way the light elements seen by XMM-Newton, speeding away from the core, could be produced. So the source of the gamma ray burst is a supernova and not a neutron star collision."

He added, "The confirmation by XMM-Newton that gamma ray bursts are associated with supernovae therefore brings scientists closer to understanding the process that leads to the burst itself."

The UK has taken a leading role in XMM-Newton, as Prof. Ian Halliday, Chief Executive of the Particle Physics and Astronomy Research Council explains: "From the development of the XMM-Newton concept, the UK has been taking a strong role, with the University of Leicester leading the way on the EPIC instrument and on the Survey Science Centre consortium which processes all the data recorded. The Mullard Space Science Laboratory built the Optical Monitor telescope on XMM-Newton, which also captured the fading optical signature of the gamma ray burst."

Prof. Ian Halliday adds "These latest findings will be tested by SWIFT, a NASA-led space mission that scientists from the University of Leicester and Mullard Space Science Laboratory are helping to build. Once it is launched in autumn next year (2003), SWIFT will study over 1000 gamma ray bursts, spotting them rapidly, then automatically turning two much more sensitive telescopes to study these events. This means that the resulting X-rays can be studied mere seconds after the event, instead of the hours presently required to maneuver existing space-based telescopes with instructions from the ground."

At 3.9 tons and 10 meters long, the X-ray Multi-Mirror (XMM)-Newton is the biggest and most sensitive X-ray telescope ever to be placed into orbit. It is able to study extremely faint X-ray sources from stars and galaxies in the most distant parts of the Universe.

EPIC, the European Photon Imaging Camera, built by an international team of European scientists led by Dr. Martin Turner of the University of Leicester, uses silicon Charge Coupled Devices to provide simultaneous X-ray images and spectra so that the chemical composition of distant X-ray sources can be studied.

(Reference: J.N. Reeves, D.Watson, J.P.Osborne, K.A.Pounds, P.T.O`Brien, A.D.T.Short, M.J.L. Turner, M.G. Watson, K.O.Mason, M. Ehle & N. Schartel, Nature, April 4, 2002.)

Related image:

The three X-ray telescopes of the XMM-Newton satellite stare into deep space. Credit: ESA/Ducros

[Contact: Dr. James Reeves, Dr. Julian Osborne, Professor Keith Mason, Peter Barratt]

05-Apr-2002