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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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Fig. 3 shows the gamma-ray spectrum obtained from GRB 090510. With this data, it was found that the low-energy gamma-ray is expressed by the broken power-law function while the high-energy gamma-ray needs “additional power-law function.EGRB 090510 is the first case where the constituent of “additional power-law function,Ewhich was also observed by the Compton satellite in the past, was clearly identified by the Fermi satellite. “Additional power-law functionEis also seen in several other gamma-ray bursts observed by the Fermi satellite. In conclusion, the Fermi satellite suggests that the high-energy gamma-ray is different from the low-energy gamma-ray in behavior in terms of both temporal and spectral aspects.

Figure 3
Figure 3. Count rate spectrum of GRB 090510 obtained by the Fermi satellite
The inserted panel shows the best fit model obtained.

The cause of the difference is partly because radiation source of the high-energy gamma-ray is the inverse Compton scattering and accelerated protons. In addition, another possibility is suggested that the source would be closely related to the afterglow phenomenon. It is expected that, as observations increase hereafter, we can approach more closely the radiation source and the radiation area’s state.

With GRB 090510, we were also able to get new findings on the kinematics of the ultrarelativistic jet. The highest energy gamma-ray detected by the satellite reached about 31 billion electron volts, which appears to be the highest level so far. It was proved that, in order for gamma-ray with such energy to escape from the radiation area without electron positron pair production reaction, the jet of the radiation area needs to move with a tremendous velocity of more than 99.9999% the speed of light (more than 1,000 in Lorentz factor) towards our line of sight. Questions such as, “do all gamma-ray bursts have a jet of this magnitude?Eand “if so, what is its generation mechanism?Eneed to be clarified with further observations.

Fermi satellite findings from GRB 090510 are not limited to the astronomical field. The high-energy gamma-ray with 31 billion electron volts was detected only 0.83 sec after the occurrence of the low-energy gamma ray. Meanwhile, some of the “quantum gravity theoryEintegrating the gravity theory and quantum mechanics predicted that, having traveled 7.3 billion light years, the arrival of the gamma-ray with 31 billion electron volts should be more delayed. Our observation result contradicted this prediction. This means that the observation result succeeded for the first time in posing directly severe restriction to the frame of the quantum gravity theory. Thus, the satellite’s findings also had a great impact on the field of fundamental physics.

A gamma-ray burst is a celestial phenomenon with a variety of faces. Depending on the burst, its brightness curve and spectrum vary largely. In this context, some characteristics common to high-energy gamma-ray are beginning to be revealed by the satellite. They may offer clues to elucidate gamma-ray radiation mechanism. It is expected that more gamma-ray bursts will be detected by the Fermi satellite. I believe that it is important to investigate and compare the natures of various gamma-ray bursts, not just to catch a “big fishElike GRB 090510 introduced here. Thus, we continue to watch carefully data from the Fermi satellite today so as not to miss any gamma-ray bursts.

Masanori OHNO

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