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The Forefront of Space Science

Physics of Fermi Acceleration Explored by Fermi Gamma-ray Space Telescope
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Direct evidence of high-energy proton

We can approach the gamma-ray emission mechanism using X-ray observation results such as the case of Tycho's supernova remnant. A more direct approach is to identify the emission mechanism from the gamma-ray spectral shape. Because meson decay gamma-ray shows a characteristic spectral shape, we were able to successfully identify meson decay gamma-ray by the Fermi satellite, providing us with direct evidence of proton acceleration.

Fig. 3 shows an example of a gamma-ray spectrum. The intensity of gamma-ray emission rapidly drops under several hundred Mega eV, which is the spectral shape peculiar to meson decay gamma-ray. This spectral structure reflects the hadron physics for the generation of neutral meson of rest mass energy 135 Mega eV and is universal almost independently of the gamma-ray radiation environment. From our gamma-ray observation results, we discovered strong evidence that cosmic rays comprised mainly of protons are accelerated in the shock wave of the supernova remnant. Since our results only observed the supernova remnant interacting with the molecular cloud, we therefore need to continue the same analysis on other types of supernova remnants.

Figure 3
Figure 3. Gamma-ray spectrum of the supernova remnant W44
Data points show the gamma-ray spectrum of supernova remnant W44 measured by Fermi satellite. Dashed line is a model of meson decay gamma-ray while dotted line is a model of bremsstrahlung radiation by relativistic electron. The spectral structure peculiar to meson decay gamma-ray is seen in the spectral data of less than several hundred Mega eV retrieved by the Fermi satellite. This indicates that the decay of meson produced by the high-energy proton is a major gamma-ray radiation mechanism.

Not limited to supernova remnants, the phenomenon of high-energy particle acceleration in celestial bodies has been observed through high-energy electrons. We believe that our achievement is significant in astrophysics because the observation of high-energy protons became possible for the first time.

Expectations for ASTRO-H

Although I have mainly discussed our research on gamma-ray observation above, X-ray observation also plays an important role in the research of particle acceleration in shock waves. Japan leads the development of the next-generation X-ray astronomical satellite, ASTRO-H, in which the author also participates as a team member. The X-ray micro-calorimeter to be installed on ASTRO-H will be a so-called nondispersive spectrometer. This enables ultra-high resolution X-ray spectral analysis, which has previously been impossible to apply to spatially-extended celestial objects such as supernova remnants. It is expected that new achievements including measurement of ion temperature and turbulence and/or discovery of deviation from Maxwell distribution could be attained through X-ray diagnosis of thermal plasma. The launch of the ASTRO-H is planned for 2015. The satellite should bring many breakthroughs in various fields, not limited to supernova remnants.

(Yasunobu Uchiyama)

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