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The Forefront of Space Science

Cosmic Rays Accelerated by Supernova Remnants
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What is the source of very high-energy gamma ray?

When discussing cosmic-ray acceleration by supernova remnants, observation of very high-energy gamma ray (light with energy of about 1012 electron volts) is indispesable. Epoch-making results in research on particle acceleration of celestial bodies has been provided continuously by observing very high-energy gamma ray with the stereo-type atmospheric Cerenkov Telescope (H.E.S.S.) developed by the leadership of the Max Planck Institute in Germany. Stereo-type atmospheric Cerenkov Telescope is a system to measure incoming direction and energy of incident gamma ray. An array of telescopes on the ground catch Cerenkov light from the cascade shower (exponential increase of electrons, positrons, and gamma ray) that is generated when gamma rays radiated from celestial bodies enter the earth's atmosphere. As shown in the left of Fig. 3, the shell structure of supernova remnant RX J1713.7-3946 is imaged in very high-energy gamma rays. This is a typical example showing the inauguration of astronomy by very high-energy gamma rays. It is said that recent results provided by the atmospheric Cerenkov Telescope would deserve the Nobel Prize.

Both X-rays and very high-energy gamma rays are radiated by particles in the highest-energy region of supernova remnants. As stated above, X-ray is synchrotron radiation. On the other hand, gamma ray is thought to be radiated by: (1) inverse Compton scattering where high-energy electrons interact with cosmic microwave background; or (2) decay of neutral pions generated by collision of high-energy protons with gas within the remnant. In either case, X-rays and gamma rays are generated by the same energy-scale particles. Thus, they are closely related and it is essential to conduct comprehensive research covering both. We are conducting in detail a comparison of X-rays and gamma rays in cooperation with the H.E.S.S. group. One example of the joint research is shown in Fig. 3 left.

As stated above, we discovered a strength change in synchrotron X-rays in the supernova remnant RX J1713.7-3946. The result strongly supports that the remnant’s extreme high-energy gamma rays are caused by the decay of neutral pions. As mentioned above, a filament with strength variation has a magnetic field of about 1 mG. From the X-ray data, it is estimated that a magnetic field of around 0.1 mG is present (average for the remnant’s entire shell). At this level of magnetic field, we are unable to explain gamma rays with the inverse Compton scattering scenario. Therefore, gamma rays can only be caused by the decay of neutral pions. We have just begun to address the “degeneracy” of the gamma-ray radiation mechanism. High-energy particle acceleration events of celestial bodies, not limited to supernova remnants, have been observed previously through high-energy electrons. It is epoch-making that we are now able to observe high-energy protons.

The new observation results below strongly support the assumption that galactic cosmic rays are accelerated by super remnant’s shock waves.

(1) A magnetic field amplified to 1 mm gauss was discovered in the supernova remnant J1713.7-3946.
(2) Cosmic ray acceleration occurred in a very disordered magnetic field.
(3) The radiation mechanism of very high-energy gamma rays is due to the decay of neutral pions caused by cosmic-ray protons.

We are now standing on the start line of research to combine observational results of X-rays and gamma rays by SUZAKU, Chandra, etc. In addition to H.E.S.S., there are atmospheric Cerenkov telescopes around the world such as MAGIC, CANGAROO-III and VERITAS. Furthermore, the gamma-ray astronomical satellite GLAST is scheduled to launch in the spring of 2008. We expect that research on high-energy particle acceleration of celestial bodies using X-rays and gamma rays will “accelerate” further.


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