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The Forefront of Space Science

Supernova Remnants Explored by X-Rays
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For the SNRs observed by historical celebrities, we successfully derived their precise proper motions based on CHANDRA's image data. For the northeast area of SN1006 recorded by Sadaie Fujiwara, we estimated that the expansion velocity is 5,000km/s and revealed for the first time that the surrounding gas density is low at 0.085cmE. Lately, TeV-level gamma ray (TeV: tera electron volt. Tera is 1012) was also detected from the SNR. The shock-wave velocity and density measured by us are expected to be an important clue to elucidate the source of the TeV gamma ray. We also succeeded in measuring the X-ray proper motion of Tycho SNR (SN1572) and Kepler SNR (SN1604) and proved that there was no discrepancy with other wavelength observation data.

Figure 1
Figure 1. Supernova remnant SN1006
Right: Supernova remnant SN1006 imaged by CHANDRA. Red corresponds to thermal radiation while blue corresponds to synchrotron radiation. Left: Radial profile of the shock wave in the northeast area. This shows how the shock wave moved forward from 2000 to 2008.

As discussed above, we can obtain various information, not just kinematics, from the measurement of an SNR's proper motion. I believe that CHANDRA and XMM-NEWTON have opened a new door on X-ray observational research of SNR. The X-ray measurement of SNR expansion has just started. I anticipate that interesting results will be obtained in a few years.

Exploring the supernova explosion based on flying-off ejecta

One of the major issues for astronomy is the mechanism of supernova explosion. A rough scenario of the explosion principle is already established, but there has been no precise simulation of an explosion performed successfully. Researchers have tackled this issue from both observational and theoretical aspects for more than 40 years, but this difficult question remains open. In general, observational research has focused on supernova, so we looked at SNR. Based on the assumption that the distribution of ejecta in SNR reflects the distribution of ejecta at the moment of explosion, we thought we should explore aspect of explosion. One merit of SNR observation is that it is possible to measure ejecta distribution in detail since they spread wide.

With XMM-NEWTON, SUZAKU, and CHANDRA, we observed the top three SNRs in X-ray luminosity in the whole sky, i.e., Puppis A, Cygnus Loop and Vela. Since these SNRs are bigger than the field of view of the X-ray telescopes, we needed to conduct targeted observations several times to see the entire SNRs. For this reason, the amount of observation data became huge, but we performed spectroscopic analysis by location carefully and in detail.

As a result, we found that the ejecta of Puppis A are densely distributed in the northeast area (Fig. 2). In addition, it was found that the bright lines of one ejecta structure are Doppler-shifted, suggesting that it is flying towards us at a speed of 2,000km/s. This fact was supported by strong ejecta evidence. Meanwhile, a neutron star located near the center of the SNR was revealed to move in the opposite direction to the one-sided distribution of the ejecta, which suggests that the ejecta and the neutron star recoiled at the explosion. We think that this recoil phenomenon is an important hint to elucidate the explosion mechanism, so we are promoting more observational research on this matter. We also found that the distribution of ejecta in Cygnus Loop is not even. A neutron star has not yet been discovered in the Loop, however, so we are looking for it.

Figure 2
Figure 2. X-ray image of supernova remnant Puppis A
The entire image is according to ROSAT data. Bright X-ray structures numbered 1 and 2 are judged to originate from ejecta. Color images of those structures taken by CHANDRA are shown in lower right. The Doppler-shift observation revealed that the ejecta 2 is flying towards us at a velocity of 2,000km/s. White and yellow arrows show the proper motion vectors (expected value during 1,000 years) of the ejecta discovered by visible light and the neutron star measured by the X-ray respectively. Yellow ellipse area is the center of the explosion estimated from the proper motion vectors. This suggests that the ejecta and the neutron star recoil from each other.

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