Gamma-ray bursts are the most powerful explosions ever detected in the Universe. They are also one of the greatest mysteries of modern astronomy, since so far no clear evidence has existed to prove what causes them.
Until now, there have been two 'prime suspects' for what makes gamma-ray bursts, the collision of neutron stars - dead, ultra-dense stars - or the death of very massive stars in supernova explosions. The new results from the XMM-Newton X-ray space telescope rule out the first hypothesis and confirm the second, at least for the gamma-ray burst that occurred on 11 December 2001.
By analysing the afterglow of the gamma-ray burst in the X-ray light, scientists produced the first ever evidence of the presence of chemical elements which were the unmistakable remnants of a supernova explosion which had occurred just a few days before. "We can now confidently say that the death of a massive star, a supernova, was the cause of a gamma-ray burst. However we still don't know how and why these bursts, the most energetic phenomena in the Universe, are exactly triggered," says ESA astronomer Norbert Schartel, a co-author of the original paper, published today in Nature.
Gamma-ray bursts were first discovered in 1967 by chance, when satellites designed to look for violations of the Nuclear Test Ban Treaty detected strong gamma-ray emission coming from sources not in the vicinity of Earth, but from outer space. They are a mystery ever since. They occur as often as several times a day but last for no longer than a couple of minutes, and there is no way to predict when or where the next burst will occur. Consequently they are very difficult to study.
For three decades it was not even known whether the explosions were close, in our own Milky Way galaxy, or far away in distant galaxies. But astronomers set up an 'alert system'. This allows them to see the 'afterglow' of the burst before it fades away, by quickly aiming their telescopes at the precise location in the sky shortly after a detector triggers the alert. It is now clear that the bursts occur in galaxies millions of light-years away.
The longest burst
Technically called 'GRB 011211', it was first detected on 11 December 2001 at 19:09:21 UT, by the Italian-Dutch satellite BeppoSAX. The burst lasted for 270 seconds - the longest one observed by the satellite. A few hours afterwards, when a first analysis confirmed that a burst had indeed been registered, the BeppoSAX team alerted the rest of the astronomical community.
ESA's XMM-Newton arrived on the 'scene of the crime' 11 hours after the original event. If XMM-Newton astronomers had reacted five hours later it would have been too late; but they were lucky and could study the afterglow when it was still 7 million times brighter (in X-rays) than a whole galaxy. It was the third time that XMM-Newton tried to pinpoint a gamma-ray burst afterglow - the results of the previous two observations were inconclusive.
On this occasion the observations revealed two important facts: first, the material in the source was moving quickly towards Earth, at 10% of the speed of light; and second, the chemical analysis of this material showed that it had to be the remnant of a supernova explosion.
"We were seeing a spherical shell of material ejected from a very recent supernova, heated by the gamma-ray burst. The fact that the material was coming in our direction means that the sphere was expanding," explains Schartel.
Silicon, sulphur, argon and calcium
XMM-Newton detected large amounts of magnesium, silicon, sulphur, argon and calcium, but very little iron. This is the kind of material a massive star would produce during its latest stages of evolution, right before exploding as a supernova.
Nuclear reactions in the stars' cores fuse light chemical elements into heavier ones, a process that generates the energy needed by the stars to shine; different elements are synthesised at each stage of the stars' evolution. The supernova explosion would have ejected this material into the surrounding environment, producing the sphere subsequently illuminated by the gamma-ray burst afterglow seen by XMM-Newton.
Astronomers could even measure the size of the sphere: 10 thousand million kilometres in radius. With that in hand, and knowing the velocity of the material, they could also estimate that the supernova explosion had occurred a few days earlier.
Such a timescale is consistent with the fact that low amounts of iron were detected, because this element forms in the material ejected by the supernova only about two months after the explosion itself.
On the other hand, the reason why the hypothesis of the neutron-star-collision can be ruled out also stems from this data. "Such an event wouldn't have expelled sufficient quantities of matter (magnesium, etc.) into the surrounding medium to explain what we see," says Schartel.
The relatively low iron abundance could also not be explained by the neutron-star-collision theory. Stars only become neutron stars after exploding as supernova, but many years - not just a few days - are needed for the object to evolve from one stage to the other.
According to Fred Jansen, ESA's XMM-Newton project scientist, "This kind of study is possible because of the unprecedented collecting area and high sensitivity of XMM-Newton. The Earth's atmosphere prevents X-rays from being detected by ground-based instruments, and no other space telescope in operation could have performed an analysis of equal quality of this gamma-ray burst afterglow. We are now at least one step closer to solving the mystery of these energetic phenomena."
However, many questions are still open in the 'case of the gamma-ray bursts'. Why are all supernovae explosions not followed by a burst? What is the precise physical mechanism triggering the burst?
In October 2002, ESA will launch a space mission to address precisely these questions. ESA's International Gamma-Ray Astrophysics Laboratory, Integral, will be the most sensitive gamma-ray observatory ever launched, able to detect radiation from the most distant violent events.
XMM-Newton, ESA's X-ray Multi-Mirror satellite, is the most powerful X-ray telescope ever placed in orbit. It was launched on an Ariane 5 rocket from ESA's spaceport in Kourou, French Guyana, on 10 December 1999. With its unprecedented sensitivity it observes the X-ray sky, helping to solve many cosmic mysteries, ranging from extremely violent and exotic processes, such as enigmatic black holes, to the formation of galaxies. XMM-Newton also observes celestial objects within our Solar System, like comets and planets.
The XMM-Newton results are reported in: 'Evidence for outflowing supernova ejects in the afterglow of Gamma Ray Burst GRB 011211' by 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 and N. Schartel, Nature, 4 April 2002.