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The Forefront of Space Science

Physics of Fermi Acceleration Explored by Fermi Gamma-ray Space Telescope
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The Fermi Gamma-ray Space Telescope launched in 2008 is a gamma-ray observation satellite that was developed by collaboration of researchers engaged in space science and elementary particle experiments. The satellite is achieving gamma-ray observation of extremely high sensitivity compared to previous satellites. The SLAC National Accelerator Laboratory in the U.S. is host to the satellite’s main detector, Large Area Telescope (LAT). Its development and operation are carried out by international cooperation of Japan, U.S. and Europe led by SLAC. The LAT is detecting gamma-rays from various celestial bodies, and greatly advancing gamma-ray astronomy.

Until March 2013, the author was engaged in research on cosmic gamma-ray observation at the SLAC National Accelerator Laboratory: in particular, the acceleration phenomenon of high-energy particles in collisionless shock waves in the universe. In this article, I would like to introduce our research on particle acceleration using the Fermi satellite with the focus on gamma-ray observation of supernova remnants. I will also mention expectations for the next-generation X-ray astronomical satellite, ASTRO-H, which is under development led by Japan with my participation.

Particle acceleration in the universe

High-energy particles and cosmic rays fly to the earth from far away in the universe. The question of how and where they are produced remains unsolved. The widely accepted theory is that cosmic rays with the energy of less than several thousand Tera eV (Tera is 12th power of 10) are created by collisionless shock waves of supernova remnants. When a supernova explodes, the fierce death of a star, its outer layer expands into interstellar space at supersonic speed to form a supernova remnant. (See the paper written by Satoru Katsuda in the Forefront of Space Science, Aug. 24, 2012). Collisionless shock waves driven by expanding ejecta from the explosion accelerate the high-energy particles.

Meanwhile, the generation of high-energy particles such as cosmic rays is observed in a wide variety of celestial bodies from stars to galaxy clusters, although the actual source of cosmic rays remains unclear. In many cases, however, shock waves are thought to be sites of particle acceleration. Therefore, understanding the mechanism of particle acceleration in shock waves is the foremost theme of astrophysics.

Fermi acceleration

The acceleration mechanism of particles in collisionless shock waves is elaborate, and is known as “Fermi first order accelerationEor “diffusive shock accelerationE(Fig. 1). This is the mechanism whereby the charged particles, which teeter drunkenly as they are scattered by magnetic waves, travel back and forth repeatedly on the front and back of the shock-wave plane and gradually increase their energy. Enrico Fermi, a prominent 20th century physicist, proposed the statistical particle acceleration mechanism in interstellar space. Later, the mechanism discussed by Fermi was applied to the shock wave. We can see that electrons are accelerated up to tens of Tera eV because, as shown in Fig.1, the synchrotron X-rays are radiated from the shock wave of the supernova remnant. Even the energy of proton beams produced by the Large Hadron Collider (LHC) detecting Higgs boson, is merely 7 Tera eV. Thus, it is surprising that the natural accelerator easily surpasses the LHC.

Figure 1
Figure 1. Tycho's supernova remnant and conceptual illustration of Fermi acceleration
Left: Tycho's supernova remnant shot by X-ray satellite Chandra. Blue color corresponds to synchrotron radiation while other colors show distribution of thermal radiation.
Right: Conceptual illustration of Fermi first-order acceleration in the shock wave. High-energy particles (cosmic rays) increase their energy every time they go back and forth on the shock-wave plane.

Although research on Fermi acceleration has greatly advanced recently, many issues remain unclear both theoretically and observationally, impeding understanding of the origin of cosmic rays. In particular, the “entranceEand “exitEof the acceleration mechanism of the particles remains a major challenge (Fig. 1). The “entranceErefers to how the thermal particles enter into the process of Fermi acceleration. Expressing it in terms of the LHC, it is a question of how to produce the incident beam. Since this is not theoretically understood, it is still unclear what volume of high-energy particles is produced by shock wave. The “exitErefers to how the accelerated particles finish the Fermi acceleration process. This is key to understanding the maximum attainable energy and to verify the assumption that the supernova remnant is the source of cosmic rays. This issue becomes more complicated by the effect of the turbulent magnetic field’s amplification on the exit and the influence of neutral particles in the interstellar gases. These issues are being pursued by both theoretical and observational approaches. This article discusses the gamma-ray observation approach.

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