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TOP > Report & Column > The Forefront of Space Science > 2014 > X-ray Precision Spectroscopic Observation of the Supernova Remnant

The Forefront of Space Science

X-ray Precision Spectroscopic Observation of the Supernova Remnant
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The Supernova Remnant

Massive stars will undergo a supernova explosion in the final stages of evolution, during which almost all of the stellar material will be sprinkled into the interstellar space. The energy of the explosion is so tremendous that about 10 days after the explosion, the maximum light intensity is comparable to the entire one of the galaxy, and it is even possible to be observed in a supernova in the far-off distance. The scattering stellar materials will spread into the interstellar space at a supersonic speed (over 1000 Mach), which can reach 10% of the speed of light. Strong shock waves will be generated in the front, so the interstellar materials that are involved will be heated to a very high temperature (over 1 million degrees) and will be in the plasma state where the electrons and atomic nuclei are unbound. On the other hand, when the amount of the involved interstellar materials increases, the stellar materials at the outside will decelerate. Freely expanding stellar materials from the inside collide with the interstellar material at supersonic speed and will cause shock-wave heating. So the remains of the supernova explosion will form a two-layer structure consisting of interstellar materials and stellar materials, and become a huge high-temperature plasma cloud. This is a celestial body called the Supernova Remnant (SNR). It is known that there are about 400 supernova remnants in our galaxy and the galaxies nearby.

The SNR is not only glaring and attractive, but it is also playing an important role in modern astronomy. For example, elements except hydrogen and helium didnít exist from the beginning of the universe. They were generated inside the stellar or during the supernova explosion, and were spread to everywhere in the universe through the SNR. Therefore, by investigating composition ratio and abundance of the elements, we can get the key towards the chemical evolution history of the universe. Furthermore, the mysterious high-energy particles, "cosmic rays", which are flying everywhere in the space, are considered to be accelerated by the shock wave of the SNR, so researches are being conducted strenuously to reveal the acceleration mechanism. It is also a valuable laboratory where we can observe plasma with an extreme high temperature in the non-equilibrium state (transient state), which is difficult to be produced in laboratories on the ground. In addition, in the near future, it is expected to directly detect the gravitational waves (distortion of the space-time predicted by the general relativity theory) caused by the supernova explosion, so the researches on the SNR and supernova are being more and more important.

Spectroscopic Observation of the SNR by X-ray

The SNR can be observed at various wavelengths from the radio waves to gamma rays, but as mentioned above, it is in a high-temperature plasma state, so in general it will emit X-rays most efficiently. The X-ray radiation contains many emission lines (the characteristic X-rays) from the higher ionized ion, a heavy element. By extracting the physical information such as elemental composition ratio, mass, temperature and speed through analysis, we can directly verify the nucleosynthesis models or diagnose the non-equilibrium plasma. Furthermore, we can explore the structure of the explosion by extracting and analyzing the spectrum for each location.

Spectroscopic observation (getting the energy spectrum by measuring the energy of each X-ray photon with high accuracy) is fundamental for such researches based on the emission line analysis. Japan has been leading the spectroscopic observations of the SNR in the past 20 years since the launch of the X-ray astronomy satellite ASCA, where for the first time in the world an X-ray CCD camera was installed. The X-ray CCD camera was an excellent camera and spectrograph combining the energy resolution (E/ΔE~10@λ=22.1Å [1]) and the spatial resolution. It succeeded in separating the emission lines from the ions in different elements and different ionization states, and achieved a lot of important results. However, we found that the X-ray CCD camera could not fully separate the emission lines. To separate the major emission lines one by one, we need one more order of magnitude higher spectral capabilities. In order to realize this, development of the next generation X-ray precision spectrograph "Micro Calorimeter" has been promoted worldwide. Japan has been ready to make the first move, and the micro calorimeter (Soft X-ray Spectrometer:SXS) has been scheduled to be installed on the next X-ray astronomy satellite ASTRO-H before others.

[1] λ represents the wavelength of light, and 1Å (Angstroms) means 100 billionth of 1m. The X-ray is light with a wavelength of about 0.1〜100Å.

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