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The Forefront of Space Science

Cosmic Rays Accelerated by Supernova Remnants
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Origins of cosmic rays

About 100 years ago mankind first noticed the existence of mysterious high-energy particles that came to the earth from far outer space. They are high-energy charged particles (mostly protons) called “cosmic rays” which fly around in space. Thanks to recent advances in radio, X-ray and gamma astronomy, we, astrophysists, are about to discover the role that these high-energy particles or cosmic rays play in the universe. At the same time, we have to admit that we are troubled and annoyed by problems in understanding the particles and their role.

Since the discovery of the cosmic ray, the conventional theme has been its origin: where and how are cosmic rays created and how do they come to the earth? For cosmic rays originating in our galactic system (galactic cosmic rays), the main theory is that they are generated and become very high-energy particles as a result of particle acceleration to almost light speed by supernova remnant’s shock waves. A key observational result to settle this issue was obtained by X-ray observations of a young supernova remnant by two X-ray astronomical satellites ESUZAKU and Chandra. This article reports the latest findings of the observation and our analysis.

Supernova remnants

All heavy stars in the universe are fated to die fiercely Ea supernova explosion caused by gravitational collapse. After the supernova explosion, black holes or compact objects like neutron stars are left in the center of the stars. Meanwhile, the star’s outer layer becomes a blast wave and expands at supersonic speed toward interstellar space (substances released by supernova explosions are called “ejecta,” which is in fact the source of all things on the earth). The ejecta expansion causes shock waves in interstellar space (external shock waves) just like supersonic jet planes. The blast’s kinetic energy transforms to thermal energy and heats interstellar gas to 1 to 10 million deg. C. On the other hand, shock waves also arise inside the ejecta (internal shock waves) and heat the ejecta itself to about 10 million deg. C (Fig. 1). The high-temperature gas radiates X-rays which shine as “supernova remnants.”

Figure 1
Figure 1. Conceptual illustration showing structure of young supernova remnant. High-temperature plasma emitting X-rays comprise two layers of the shell. The outer shell is interstellar gas heated by external shock waves while the internal is ejecta (material released by supernova) heated by internal shock waves. Electrons “heated” by shock waves radiate thermal bremsstrahlung X-rays and a very few electrons “accelerated” to high-energy level emit synchrotron light that spreads from radio waves to x-rays.

The youngest supernova remnant discovered until now in our galaxy is Cassiopeia A, a remnant of a supernova explosion that occurred about 340 years ago. The left of Fig. 2 shows the current figure of Cassiopeia A shot by NASA’s Chandra X-ray satellite. Chandra has an X-ray reflecting telescope with excellent spatial resolution exceeding one-arc-second, so the beautiful remnant figure is clearly imaged. The ejecta heated by internal shock wave radiates X-rays mainly. The red and blue in the X-ray image trace bright lines of silicon and iron respectively, which were synthesized by nuclear fusion reaction inside the star and/or at the time of the explosion.

Figure 2
Figure 2. Left: X-ray image of supernova remnant Cassiopeia A shot by Chandra (red: silicon line 1.7 - 2.2keV, green: continuum 4 - 6keV, blue: iron line 6.4 - 6.9keV). Right: Broad X-ray spectrum of entire Cassiopeia A observed by SUZAKU. “Non-thermal component” clearly exists in the region above 10keV. Our research revealed that this is synchrotron light (refer to Figure 1).

The X-ray astronomical satellite SUZAKU carries two types of detector: X-ray CCD camera “XIS” and hard X-ray detector “HXD.” The right of Fig. 2 shows the broadband X-ray spectrum, or X-ray energy distribution, of Cassiopeia A measured by SUZAKU. As you can see, SUZAKU allows us to perform high-sensitivity observation covering a wide energy band from soft to hard X-ray. In addition to the bright lines of silicon, sulfur, and iron elements of the ejecta, the spectrum contains continuous X-rays caused by thermal bremsstrahlung (X-ray radiation generated when high-speed thermal electrons receive Coulomb's force of nuclei). Moreover, the third component, so-called “non-thermal X-ray,” appears in the higher-energy side. This component is indeed key to uncover the mystery of cosmic rays.

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