NEWS | Fondamental search
The Fermi telescope discovers the most violent explosion ever observed in the Universe
© NASA/Fermi LAT Collaboration
The most violent gamma-ray burst ever observed has been detected by the Fermi Gamma-ray Space Telescope. The total energy released, the ejection velocity of the explosion and the extremely high energy of the light emitted are exceptional.
Published on Tuesday 3 March 2009
Five French teams from IN2P3/CNRS(1), CEA(2) and the Université Paris Diderot(2), and INSU/CNRS(3) took part in analyzing and interpreting these results, which were published in the 19 February 2009 issue of the journal Science Express.
Gamma-ray bursts are sudden blasts of gamma rays that can last anything from a fraction of a second to a few minutes, usually followed by a weaker afterglow lasting several days. Most bursts are probably caused by the hearts of very massive stars near the end of their lives collapsing into a black hole. The formation of the black hole is accompanied by a violent ejection of plasma which expels part of the star's matter in the form of a jet moving very nearly as fast as light(4).
The GRB 080916C burst appeared in the constellation Carina and was recorded by Fermi's two instruments. Because of their very broad energy coverage (from 10 000 eV to over 10 billion eV), these instruments form a gamma-ray observatory that is the only one of its kind in the world. The Gamma-ray Burst Monitor detector provided the initial alert at low energies, and the Large Area Telescope was used to study it at high energies. The most extreme radiation emitted by the source carried 30 billion times more energy than visible light.
A third ground-based instrument, the Gamma-Ray Burst Optical/Near-Infrared Detector (GROND), detected GRB 080916C during its afterglow phase. Such observations at different wavelengths enabled its distance to be estimated at around 12.2 billion light years, which makes it extraordinary because of its huge power. The scientists estimated (5) that the blast exceeded the power of 8000 supernovae, and was equivalent in energy(6) to 5 solar masses emitted in under 60 seconds in the form of X-rays and gamma rays. Within the jet, matter was expelled at over 99.9999 % of the speed of light. All these characteristics make this gamma burst the most violent celestial object ever observed.
Fermi's scientists also observed a delay of around 5 seconds in the arrival times of gamma rays with different energies. This time-lag, which has already been observed in another burst, may mean that low and high energies are produced in different regions of the jet, and thus make it possible to better understand the mechanisms of particle acceleration at work.
Gamma rays are produced by the most extreme phenomena in the Universe. The celestial objects associated with such phenomena, involving unimaginable amounts of energy, are at the origin of the acceleration of very high-energy particles. The list of such objects includes active galactic nuclei, the remains of supernovae, pulsars and gamma-ray bursts. Since Fermi was launched in summer 2008, it has pushed back the limits of observation of the distant Universe.
NASA's Fermi Gamma-ray Space Telescope was developed in collaboration with the US Department of Energy, with major contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the US.
(1) LLR : Laboratoire Leprince-Ringuet (CNRS/École Polytechnique), CENBG : Centre d'études nucléaires de Bordeaux-Gradignan (CNRS/Université de Bordeaux 1), LPTA : Laboratoire de physique théorique et astroparticules (CNRS/Université Montpellier 2).
(2) IRFU : Institut de recherche sur les lois fondamentales de l'Univers, Service d'Astrophysique (SAp), Laboratoire AIM (CEA/Université Paris Diderot/CNRS) and Service d'Electronique des détecteurs et d'Informatique (Sédi), Saclay.
(3) Centre d'Étude spatiale des rayonnements (CNRS/Université Toulouse 3).
(4) The initial emission probably results from shock waves that accelerate the particles within the jet, and the afterglow is likely to be caused by the slowing down of the jet as it travels through the surrounding interstellar clouds.
(5) Since the size of the cone of the jet of matter is extremely hard to measure, scientists often calculate the power of gamma-ray bursts by assuming that the energy was released in all directions.
(6) According to Einstein's famous formula E = mc2.
Bibliography
A. A. Abdo, et al., « Fermi Observations of High-Energy Gamma-Ray Emission from GRB 080916C », Science Express, 19 February 2009
Envoyer cet article par e-mail
* Champ obligatoire