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Orion Nebula binary star resolved by VLTI

... astronomers have plunged into the heart of the Orion Trapezium Cluster to produce the sharpest image ever obtained of the young double star Theta 1 Ori C...

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Hubble finds hidden exoplanet in archival data

...A powerful image processing technique may allow astronomers to seek out exoplanets that could be lurking in over a decade's worth of Hubble Space Telescope data...

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100 hours of astronomy

...Another great Cornerstone Project of the International Year of Astronomy kicks off this week with 100 hours of astronomy - the largest single science public outreach event ever organised...

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Video archive

STS-120 day 2 highlights

Flight Day 2 of Discovery's mission focused on heat shield inspections. This movie shows the day's highlights.


STS-120 day 1 highlights

The highlights from shuttle Discovery's launch day are packaged into this movie.


STS-118: Highlights

The STS-118 crew, including Barbara Morgan, narrates its mission highlights film and answers questions in this post-flight presentation.

 Full presentation
 Mission film

STS-120: Rollout to pad

Space shuttle Discovery rolls out of the Vehicle Assembly Building and travels to launch pad 39A for its STS-120 mission.


Dawn leaves Earth

NASA's Dawn space probe launches aboard a Delta 2-Heavy rocket from Cape Canaveral to explore two worlds in the asteroid belt.

 Full coverage

Dawn: Launch preview

These briefings preview the launch and science objectives of NASA's Dawn asteroid orbiter.

 Launch | Science

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Integral dissects bright gamma-ray burst



Posted: 03 April, 2009

Integral has captured one the brightest gamma-ray bursts ever seen, allowing astronomers to probe the mechanics of the initial stages of such powerful stellar explosions.

Gamma-ray bursts (GRBs) are the most luminous electromagnetic events that occur in the Universe, and represent the death cries of rapidly rotating, high-mass stars as they collapse into a black hole. The blast from the catastrophic event known as GRB 041219A arrived at the Earth on 19 December 2004, where ESA’s Integral satellite saw the 500 second-long burst rise to extraordinary brilliance.

GRB 041219A flared dramatically over a short period, allowing astronomers to probe the mechanisms producing gamma-ray bursts. Image: ESA (Illustration by AOES Medialab).

“It is in the top one percent of the brightest GRBs we have seen,” says Diego Götz, CEA Saclay, France, who headed the investigation. The explosion threw off matter at velocities close to the speed of light, and may prove to be one of the most important GRB events yet.

Its impressive brightness allowed the astronomers to extract the polarisation of the gamma rays, that is, the preferred direction in which radiation waves oscillate. The results revealed that the gamma rays were highly polarised and varied enormously in level and orientation. The polarisation is directly related to the structure of the magnetic field in the jet, so it is one of the best ways for astronomers to investigate how the central engine produces the jet.

There are a number of scenarios that propose mechanisms to allow this to happen. One school of thought is that the jet carries a portion of the central engine’s magnetic field into space, while another involves the jet generating the magnetic field far from the central engine. A third idea is an extreme case in which the jet contains no gas, just magnetic energy. A fourth scenario describes the jet moving through an existing field of radiation.

Artist impression of the centre of a dying star collapsing before imploding. The blast from a Gamma Ray Burst is thought to be produced by a jet of fast-moving gas that bursts from near the central engine; probably a black hole created by such a collapse of the massive star. Image: NASA/Dana Berry.

In the first three scenarios, the polarisation is generated by what is called synchrotron radiation, whereby the magnetic field traps electrons and forces them to spiral, releasing polarised radiation. In the fourth scenario, the polarisation is imparted through interactions between the electrons in the jet and photons in the existing radiation field.

Götz believes that the observations of GRB 041219A favour a synchrotron model and in particular, the first scenario, in which the jet propells the central engine’s magnetic field out into space. “It is the only simple way to do it,” he comments.

The next step will be to examine the polarisation of other GRBs, to see if the same mechanism applies to all GRBs. But most GRBs are too faint; Integral can only record the polarisation state of gamma rays if a celestial source is as bright as GRB 041219A. “So, for now we just have to wait for the next big one,” he says.

The results of this survey will be published in the journal Astrophysical Journal Letters.