BY DR EMILY BALDWIN
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.