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  Current Research Highlight :: April 2007 all highlights

How to produce a burst of gamma-rays

Gamma-ray bursts (GRBs) are the most luminous events known in the universe since the Big Bang. They are flashes of gamma rays, appearing at random locations in the sky and at random times. Scientists at the Max Planck Institute for Astrophysics have found a way to explain how these flashes are produced by studying how matter interacts with radiation under extreme conditions.

Fig. 1: The simulated spectrum of radiation expected from a magnetically powered GRB. One important quantity of the flow is the luminosity-to-mass-flux ratio η; a measure of its "cleanness". For high values of η the spectra peak at around 1000 keV. The flat tail above this peak results from scattering of photons off hot electrons in the flow. For low η, the magnetic annihilation stops below the photosphere. The resulting spectrum is then quasi-thermal.

Fig. 2: Example of how observations can tell us something on the properties of the fast moving material. A gamma-ray-burst light curve consists of many pulses. Their width decreases with photon energy. Observations also show that the peak of the radiation spectrum increases with the luminosity of the burst. Both of these behaviors are reproduced by the theoretical calculations if the most powerful flows are also the "cleaner" ones. The plot shows the predicted narrowing of the pulses with increasing photon energy.

Gamma-ray bursts are detected by satellites about once a day. The gamma-ray emission lasts from milliseconds to many minutes and is followed by the "afterglow" emission at lower energies (X-rays, ultra-violet, optical, infra-red, radio) before they dissappear for ever.

The majority of observed GRBs appear to be the end result of the evolution of very massive stars (40 times more massive than our sun, or more) when the core of the star collapses into a black hole. A specific subclass of GRBs (the shorter duration bursts) appears to be due to another process, possibly the collision of two neutron stars orbiting each other.

It has been known for many years that ejection of matter at velocities very close to the speed of light is necessary to explain the burst of gamma rays. The acceleration of matter to these high speeds and the emission mechanisms responsible for the burst are still somewhat of a mystery. Theoretical studies now show that magnetic fields may well be the key element for the explanation of gamma-ray bursts (linkPfeil.gif Research Highlight February 2002). The annihilation of magnetic field lines causes not only the strong acceleration of material, but also heats it up so that it can glow.

D. Giannios and H. Spruit at the Max Planck Institute for Astrophysics have looked more closely how radiation interacts with matter in these strongly magnetized flows. Their study points towards a specific distance from the explosion center as the most promising location to produce the burst. This is the distance where radiation stops interacting with matter (the so called photosphere). At this radius, the energy released by magnetic annihilation heats the flow. Radiation that is carried with the flow is scattered by the hot material and gains energy. Numerical simulations show that the spectrum of radiation from this hot flow is very similar to the observed gamma-ray spectra.

The calculations directly connect the properties of the fast moving material to the emitted spectrum. This is a very interesting result since the study of the observed properties of the burst can be used to make inferences on the how the ejection of the flow actually takes place. There are observations of thousands of bursts that can be used to shed light on this poorly understood phase of gamma-ray bursts.

Dimitrios Giannios


D. Giannios,
"Prompt emission spectra from the photosphere of a GRB",
A & A, 457, 763(2006);

D. Giannios and H. C. Spruit,
"Spectral and timing properties of a dissipative GRB photosphere",
A & A, in press; linkPfeilExtern.gif astro-ph/0611385


Dr. Dimitrios Giannios, Max Planck Institute for Astrophysics,
Tel. 089/30000 2269, Email: giannios

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