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TOP > Report & Column > The Forefront of Space Science > 2005 > GEOTAIL Satellite - Detailed Story of Astrophysical Gamma-ray Observation

The Forefront of Space Science

GEOTAIL Satellite - Detailed Story of Astrophysical Gamma-ray Observation
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Magnetic reconnection affects magnetar’s flare

It is a widely accepted theory that the real identity of SGR1806-20 is a neutron star (magnetar) with an ultra-strong magnetic field up to about 1 quadrillion gauss. Magnetars are called Soft Gamma-ray Repeaters (SGR) because they are always releasing a little magnetic field energy intermittently to radiate relatively low-energy gamma rays. Until now, three magnetars (SGR0526-66, SGR1900+14, and SGR1806-20 in the main figure) have caused huge flares. It turned out that the flare this time was 100 times bigger than the previous two.

It is guessed that magnetars cause explosive releases of magnetic field energy as huge flares, far bigger than usual, once in a few decades. The details of the huge flare remain unknown, however. To study its mechanism, we need to know the changes in gamma-ray power just after flare eruption. LEP data are very valuable for this reason and must make a great contribution by providing clues to build a theoretical model in the future. In particular, the time scale of the light curve for about 60 milliseconds between 0 and 200 milliseconds in which fluctuations appear (shown in the upper graph in Fig. 3) is thought to reflect the time scale of the elementary steps in the magnetar’s energy release. According to a commonly accepted theory, magnetic reconnection probably affects the elementary steps. If the theory is correct, GEOTAIL is the first satellite to observe magnetic reconnection of both the earth’s magnetosphere and another celestial body.

At the end of March, it was decided that our data would be published in “Nature” along with other gamma-ray satellites’ results. Thus, our gamma-ray observation this time came to an end.


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