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The Forefront of Space Science

Cosmic Rays Accelerated by Supernova Remnants
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Catching a moment of cosmic ray acceleration

The Chandra and SUZAKU satellites observed RX J1713.7-3946, a supernova remnant with strong “non-thermal X-ray” radiation. Chandra’s X-ray observation results were surprising. A comparison of the X-ray images of the remnant’s outermost layer shot in 2000, 2005 and 2006 revealed that the aspect of non-thermal X-ray radiation changed year by year as shown in Fig. 3. The filamentary radiation regions Ewe guess these to be a reaction to external shock waves - occasionally appeared and disappeared. The observation results show explicitly that non-thermal X-ray is synchrotron radiation. It is caused by spiral motion in the magnetic field of a very few electrons that have gained extremely high energy (i.e., extremely accelerated) by shock waves. These high-energy electrons are cosmic rays. It is thought that the filament darkens because high-energy cosmic-ray electrons have lost energy by synchrotron radiation. Meanwhile, the newly appearing filaments mean that acceleration of cosmic rays occurs in these locations. Thus, for the first time, we successfully caught the phenomenon of cosmic-ray acceleration in real time. The theory that cosmic rays are accelerated by shock waves from supernova remnants has been widely advocated by researchers. This time we were finally able to witness the “moment” of cosmic-ray acceleration.

Figure 3
Figure 3. Left: X-ray image (colored) by Chandra and gamma ray image (contour) by H.E.S.S. of the western area of supernova remnant RX J1713.7-3946. With joint analysis with the H.E.S.S. team, it is clear that the spatial distribution of X-rays and gamma rays coincides.
Right: Enlarged images of boxes “b” and “c” in the northwest area. Yearly changing X-ray filaments are seen. Aspect of cosmic ray acceleration was imaged in real time for the first time. Reprint from Uchiyama et al. Nature 449, 576 (2007)

To explain the observed annual-scale change in strength, the magnetic field must be amplified to 1 milli-gauss in strength, 100 times that of interstellar space. It is believed that this is caused by the magnetic field’s non-linear amplification coupled with cosmic-ray acceleration. This provides us with a new clue to understanding the basic characteristics of shock-wave acceleration in the universe.

By analyzing Chandra’s aforementioned X-ray data of Cassiopeia A, the supernova remnant, we also found a temporal change in the X-ray filaments. The essential difference in the case of Cassiopeia A from RX J1713.7-3946, however, is that its filaments receive internal shock waves to shine on and off. This result is important because it suggests that high-energy cosmic-ray acceleration can be induced by a shock wave inside the ejecta, not just by an external shock wave forming in interstellar space.

SUZAKU also observed the supernova remnant RX J1713.7-3946. We confirmed that a spectrum bend Ea theoretically predicted “cut-off” - existed in the hard X-ray region. It means that the acceleration of cosmic ray electrons peaked out. In other words, an increase in net energy becomes zero due to synchrotron radiation cooling. Comparing the observation results and shock-wave acceleration theory, it is clear that cosmic-ray acceleration occurs very efficiently in fully tangled magnetic field. From two separate observation results (i.e., 0.1 to 1 milli-gauss magnetic field observed by Chandra and acceleration in fully tangled magnetic field revealed by the SUZAKU observation), we were able to obtain decisive evidence for the assumption that cosmic-ray protons can be accelerated to extremely high-energy over 1015 electron volts, and that galactic cosmic rays composed mainly of protons can be accelerated and created by shock waves from supernova remnants.

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