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New models shed light on enigmatic explosions

Max Planck Society : 23 November, 2006  (New Product)
Researchers at the Max-Planck-Institute for Astrophysics have developed new relativistic models which allow predictions of so far unknown properties of short gamma-ray bursts. Their simulations will come under scrutiny by the Swift Gamma-Ray Burst Explorer.
Gamma-ray bursts are among the most energetic and most luminous explosions in the Universe. They occur roughly once a day, last from a few thousandths of a second to a few hundred seconds, and come from all different directions of the sky. Their gamma radiation is more energetic than visible light and can be measured by satellites orbiting the Earth in space. The energy set free by the bursts in just one second is comparable to the energy production of the Sun during its whole life.

The more than 2700 observed bursts are grouped into two distinct classes, one of which are the so-called long bursts that emit gamma radiation for more than two seconds, and the other one are the short bursts with durations up to two seconds.

So far only long bursts could be observed in much detail. The detection of associated afterglows in X-rays, visible light and at radio wavelengths allowed the determination of their distances and confirmed their origin from host galaxies at large redshifts, i.e., typically hundreds of millions to billions of light years away. Until recently the source of these bursts was a mystery. But evidence has accumulated that they are death throes that accompany the catastrophic explosions which end the lives of very massive stars. A final confirmation of this conjecture was provided by GRB030329, a gamma-ray burst which was detected, by HETE, NASA's High-Energy Transient Explorer satellite.

But where does the gigantic energy come from which powers the gamma-ray burst? Scientists have coined the theory that the 'engine' is a rapidly spinning black hole which forms when the central core of a dying star becomes unstable and collapses under its own gravity. This newly formed black hole then swallows much of the infalling stellar matter and thereby releases enormous amounts of energy in two 'jets'. These expand 'highly relativistically', i.e. with almost the speed of light, along the rotation axis of the star. Before they break out from the stellar surface, they have to drill their way through thick layers of stellar material, thus getting collimated into very narrow beams with an opening angle of only a few degrees. Indeed, observations not only confirm the origin of long gamma-ray bursts from exploding massive stars, but also provide evidence that the gamma emission comes from narrowly collimated, ultrarelativistic jets with velocities of more than 99.995 per cent of the speed of light.

Rotating, growing stellar mass black holes are also born in other cosmic events, for example in the violent mergers encountered by binary neutron stars or a neutron star and a black hole after hundreds of millions of years of inspiral, driven by the emission of gravitational waves. The remnant of such a catastrophy is a stellar-mass black hole sucking matter from a girding, thick torus of gas. Such events have long been considered as possible sources of gamma-ray bursts, and they are still hot candidates for bursts of the short type, which so far could not be studied by observations in the same way as bursts from dying stars.

Researchers at the Max-Planck-Institute for Astrophysics have now developed better computer models that take into account effects due to Einstein's theory of relativity. Their simulations can follow the highly relativistic ejection of matter that is caused by energy release (e.g., due to particle reactions) in the close vicinity of the black hole. The calculations confirm that short bursts have properties that are distinctively different from those of long bursts. Since the black hole, torus system is not buried inside of many solar masses of stellar material as in case of dying stars, the polar jets do not have to make their ways through dense stellar layers and quickly reach extremely high velocities. As a consequence, they are strongly collimated by the presence of the accretion torus, but their opening angles are somewhat larger than those measured for long bursts, typically around 5 to 10 degrees. The models predict that outside of these polar cones gamma emission should become very weak so that a gamma-ray burst will be observable only from one out of hundred mergers when the ultrarelativistic jet is sent towards Earth. The models also suggest that short bursts can be nearly as bright as long bursts, although their total energy release is 100 times lower.

Previous gamma-ray satellites were unable to make precise measurements for short bursts, but there is hope that these predictions can be tested soon. The Swift Gamma-Ray Burst Explorer, a NASA mission with international participation. One of its prime goals is to unravel the mysteries of the short bursts.
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