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The Forefront of Space Science

Supernova Remnants Explored by X-Rays
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Supernova and supernova remnant (SNR)

A supernova shines suddenly and brightly in the night sky. The term "supernova" was coined because it looks like a suddenly emerging new star, but it is in fact a huge explosion of stars ending their lives. The explosion energy is furious and its maximum luminosity is equivalent to that of a single galaxy. In the annals of history, there have been cases where explosions occurred near the earth and were so bright they were observed even in daylight. From the second to the 17th century, seven cases of explosions have been recorded in the history books. The brightest one among them is thought to be a supernova in the year 1006, whose brightness was reported to be a quarter of the full moon's. Records of its observation have been kept across the world. In Japan, Sadaie Fujiwara wrote about it in his Meigetsuki when he heard the rumors.

A supernova darkens to less than 1/1,000th of its peak within a year of its emergence and, eventually, observation with the naked eye becomes impossible. In the meantime, blast waves caused by explosions spread at a tremendous speed of about 10,000km/s (circling one quarter of the earth in a second!), sweeping up the surrounding interstellar gases. As a result, an extremely high-temperature plasma cloud (plasma means a state where atoms are divided into electrons and ions) is formed in outer space. This is the celestial body called "supernova remnant" (SNR). An SNR cannot be seen with the naked eye. Because observation equipment has been improved lately, however, about 300 SNRs have already been discovered even in our galactic system.

Observing an SNR with X-rays

The typical temperature of SNR plasma is from a million to tens of millions deg. C. Under such high temperatures, radiation in the visible light region is faint. But SNR shines brightly in the X-ray region (i.e., light of 1/1,000th wavelength of visible light). Thus, SNRs are not distinct objects in the visible light, but show strong presence in the hot X-ray universe.

X-rays cannot go through the atmosphere, so we use artificial satellites and/or balloons to observe X-rays from celestial bodies. Currently, three X-ray astronomical satellites, the U.S.'s CHANDRA, Europe's XMM-NEWTON and Japan's SUZAKU, are in orbit in space. All of them are using non-dispersive X-ray CCDs as focal plane detectors, which makes imaging spectroscopy possible. This observation system has been commonly employed since ASCA, an X-ray astronomical satellite launched by Japan in 1993. The observational research of SNRs has been rapidly advanced by the system with full use of its imaging spectroscopy capability. This article introduces a part of the research.

Kinematics of SNR: Direct observation of blast-wave motion

The blast waves produced by supernova explosions spread at a tremendous speed, but it is difficult to observe the scene directly because they are too distant. For example, even the nearest SNR is 1,000 light years away. Thus, when we observe its movement from the earth, it corresponds to only one arcsecond at most. Therefore, past measurement of the proper motion has been performed only in visible light and radiowave regions because of their good angular resolution. However, visible light and radiowave observation has disadvantages: only a limited area of the entire SNR is visible. So, observation by X-ray has long been desired because X-rays are strongly emitted from everywhere in an SNR. In the past, X-ray observations were performed by ROSAT of Germany and EINSTEIN of the U.S. Taking into account their spatial resolution (5 arcsecond), however, their attempts are considered to be very challenging.

Under this situation, two satellites with good eyesight, CHANDRA, boasting 0.5 arcsecond spatial resolution in performance, and XMM-NEWTON, slightly lower at about 10 arcsecond, were launched in 1999 by the U.S. and Europe respectively. Now, ten years after the launches, precise measurement of the expansion process of SNRs becomes possible by comparing images taken at the early stage of the launch and those taken later. We (research group consisting of Osaka University including the author, University of Miyazaki, NASA Goddard Space Flight Center, etc.) succeeded, for the first time in the world, in measuring the proper motion of SNRs existing near the earth.

The first SNR we measured was Vela Jr., which was discovered by ROSAT in 1998. Simultaneously, the detection of the gamma ray (half-life of 60 years) caused by the decay of 44Ti (titanium) was reported from observation by the gamma-ray astronomical satellite COMPTON. This was the second case, following Cassiopeia A, where gamma-ray emission from SNR had been detected, thus attracting a lot of interest from many researchers. Vela Jr. was estimated to be 680 years old, which was calculated by estimating the amount of 44Ti according to the observed gamma-ray intensity and, then, comparing it to the amount at its explosion as forecast by the nucleosynthesis model. In the meantime, the age of the neutron star likely to be associated with the SNR was estimated to be several thousand years old. This inconsistency in the ages remained as a question. We analyzed the image data of the northwest edge of Vela Jr., obtained four times (in 2001, 2003, 2005 and 2007) by XMM-NEWTON, and discovered that its proper motion was 0.84 0.23 arcsec/year. Moreover, based on the fact that the proper motion of SNR decreases with age, we estimated that the age of the SNR was 1,700 to 4,300 years old. Supposing that this age is correct, the generated amount of 44Ti at explosion is too large by several orders of magnitude and is unrealistic. Thus, we came to an important conclusion that there was an error in the gamma-ray detection. In fact, significant detection was not reported by re-analysis of the same gamma ray data.

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