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TOP > Report & Column > The Forefront of Space Science > 2010 > Watching Huge Explosions in the Farthest Area of the Universe. Mysteries of Gamma-ray Bursts to be Unraveled by Fermi Satellite

The Forefront of Space Science

Watching Huge Explosions in the Farthest Area of the Universe. Mysteries of Gamma-ray Bursts to be Unraveled by Fermi Satellite
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Most distant and intensive explosions

The most distant celestial event in the universe so-far identified by mankind is the “gamma-ray burst,Ean event that shines suddenly and brightly for several milliseconds to several hundred seconds in gamma-ray, an electromagnetic wave with higher energy than visible light or X-ray.

Recent observations revealed that the source of gamma-ray bursts is plasma jets with ultrarelativistic velocity which are generated at the collapse of large mass star or at the fusion of two neutron stars or two black holes existing in cosmological distant areas, and the relativistic jet is believed to face to our line of sight. These bursts draw researchersEattention because they shine brightly in the most distant currently observable universe and, therefore, are considered to be searchlights to light up the figure of the early universe. What we really want to know about the bursts, however, is still unknown. For example: “What is the gamma-ray radiation mechanism?Eand “what is generation mechanism of the ultrarelativistic jet?E

This article introduces the latest findings obtained by the Fermi Gamma-ray Space Telescope (Fermi satellite). Launched in June 2008, the telescope is an international collaborative mission by the U.S., Japan and Europe to observe high-energy gamma ray, which is key to elucidate still-unsolved mysteries in relation to the gamma-ray burst.

What is the gamma-ray radiation mechanism?

The radiation mechanism of gamma-ray bursts remains a mystery. However, for the energy range of 1,000 to 10 million electron volts* (in this article we tentatively call this range “low-energy gamma-rayE, the widely accepted theory is that the bursts shine due to synchrotron radiation, which is generated by the spiral motion of electrons accelerated by the magnetic filed in the relativistic jet. Nonetheless, some behaviors of gamma-rays over 100 million electron volts (tentatively, “high-energy gamma-rayE, which were detected by the Energetic Gamma Ray Experiment Telescope (EGRET) onboard the Compton satellite (a mission prior to the Fermi satellite), were not explainable by the synchrotron radiation theory. The high-energy gamma-ray spectrum did not agree with the broken power-law function which is observed in synchrotron radiation, but was presented by “additional power-law function.EFurther, the duration of low-energy gamma-ray radiation is just tens of seconds while the high-energy gamma-ray continues to shine for longer, in some cases, more than 90 minutes.

*One electron volt (eV) is the energy obtained when one electron is accelerated by one-volt electric potential difference.

Several theories were presented for the high-energy gamma-ray. One is that it is an inverse Compton scattering caused by interaction between photons generated by synchrotron radiation and high-energy electrons present there. Another is that it is radiation of electrons produced by the reaction of accelerated protons and gamma-ray. In any case, the high-energy gamma-ray, which shows different behaviors from the synchrotron radiation in terms of both spectrum and duration, is an important key to unravel the radiation mechanism of the gamma-ray burst. In particular, if the burst is radiation from accelerated protons, there is a possibility that gamma-ray bursts could offer great insights into the origin of cosmic rays.

The Compton satellite observations, however, detected very few cases of high-energy gamma-ray, and insufficient data for further discussion. In this situation, the Fermi satellite was launched in June 2008 as the successor to the Compton Observatory.

Figure 1
Figure 1. Fermi Gamma-ray Space Telescope (Fermi satellite)
At left is the Fermi satellite being installed on the rocket before launch. Large Area Telescope (LAT), main detector of the satellite, converts incident gamma-ray (red line) to electrons and positrons (blue lines). Then, the silicon strip detector tracks their flight paths to measure gamma-ray energy and arrival direction. (Left: ©NASA/Jim Grossmann, Right: ©NASA)

As shown in Fig. 1, a Large Area Telescope (LAT), the satellite’s main detector, converts the incident high-energy gamma-ray to electrons and positrons by electron positron pair production reaction. Then, by tracking their flight paths with semiconductor detectors called silicon strips, we can measure the gamma-ray’s arrival direction and energy in broad band from 10 million to 100 billion electron volts. The Fermi satellite also carries a Gamma-ray Burst Monitor (GBM) capable of observing low-energy gamma ray from 10,000 to 10 million electron volts. As a result, the satellite can measure gamma-rays coming from various gamma-ray bursts covering a seven-digit range of energy.

From Japan, Hiroshima University, the Institute of Space and Astronautical Science (ISAS) of Japan Aerospace Exploration Agency (JAXA), and Tokyo Institute of Technology have contributed to the development of the Fermi satellite. Especially, the silicon strip detector, the heart of the satellite, was designed and developed by the cooperation of Hiroshima University and Hamamatsu Photonics. As the quality and performance of the detector were highly assessed, the team directed the production of about 10,000 sensors. The Japan group also contributed to: the balloon experiments; 100% inspection in the assembly process; operation and monitoring after launch; and data analysis.

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