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The Forefront of Space Science

Supernova Remnants Explored by X-Rays
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For Vela, we analyzed in detail the X-ray structure expanding around it. Based on its shape, we guessed the structure was fragments of the star that had flown off like bullets at the supernova explosion. This was confirmed by evidence from our own spectroscopic analysis. In that case, how was the gas cluster formed inside the star? Some researchers think that it is caused by hydrodynamic instability arising at the supernova explosion. Possibly, it may relate closely to the mechanism of supernova explosion.

Detection of X-rays originating from charge-exchange reaction

It has been believed for a long time that the X-ray emissions from SNRs are limited to only two types, i.e., thermal bremsstrahlung and non-thermal synchrotron radiation. Recent observation by SUZAKU suggested a new view, however, that, in addition to the above two, we should not ignore X-rays originating from charge-exchange reaction.

SUZAKU performed 28 spectroscopic and imaging observations of the entire outer rim of Cygnus Loop and derived its X-ray structure and spectra. In this study, we found a line structure in the region just after the shock wave that cannot be explained by thermal radiation (Fig. 3). As a result of investigations from various standpoints on its source, we come to the conclusion that it is likely to be radiation caused by the charge-exchange reaction of H-like oxygen and neutral hydrogen. Charge-exchange reaction X-rays are frequently observed on ground experiments or in comets, but they have not been detected from SNRs. This is a very interesting result, and suggests new developments in SNR research. In the future, we need to conduct additional observation to verify our guess on the radiation and examine its scientific significance.

Figure 3
Figure 3. Spectrum drawn from the outer rim of the Cygnus Loop as measured by SUZAKU
Cross marks show data while the curve shows the thermal radiation model. The data almost reproduce the thermal radiation model, but depart from the model around 0.7keV (indicated by arrow).


This article introduced part of the X-ray observation results of SNRs using X-ray astronomical satellites, CHANDRA, XMM-NEWTON and SUZAKU. There were a number of other interesting results. Observational research by X-ray is advancing strongly and steadily. The micro-calorimeter onboard Japan's next X-ray astronomical satellite ASTRO-H is a non-dispersive spectrometer with 20 times better performance than an X-ray CCD. There is no doubt that the micro-calorimeter will advance SNR research considerably. We look forward to the new satellite and offer our sincere appreciation to all the staff engaged in design and development of the new observation equipment.

Satoru Katsuda

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