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The Forefront of Space Science

The Realization of Observing the Gamma-Ray Polarization and Studies on the Radiation Mechanism of Gamma-Ray Bursts
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The role of X-ray astronomy satellite “SuzakuE/span>

The observation of the white dwarfs

The problems of our model is that not only the weight of the white dwarf but also that plasma flow rate per unit area are necessary to measure at the same time. Therefore, if the model use a data not be observed enough, the weight of the white dwarfs cannot be obtained. “SuzakuE a X-ray astronomy satellite can be used to solve the problems.

As X-ray astronomy satellite fifth in Japan, “SuzakuEwas launched in July 2005 and also be on duty currently. “SuzakuEwas produced in international cooperation with the United States, and equipped four sets of observation system with one X-ray telescope(XRT)and one X-ray CCD (XIS) , and one Hard X-ray detector(HXD). Observation system of XIS and XRT can be use to measure the X-ray about 0.2~12keV (electron volts, 1 electron volt is energy can be wined in the potential difference of 1V per one electron), and the HXD have a capability about 10~600keV.

So far as about 20 pieces of hight-qulity data of the white dwarfs with strong magnetic field has been obtained by the “SuzakuE Fig.2 is X-ray spectrum of the white dwarfs with strong magnetic field called EX Hydrae star obtained by “SuzakuE

Figure 2
Figure 2. X-ray spectrum of the EX Hydrae by “SuzakuE
Black line comes from the front irradiation type of X-ray CCD camera (XIS), while red line comes from the backside irradiation type. And the green line comes from the hard X-ray detector (HXD). The structure shown by the arrow are the emission lines of the respective elements.


Also, X-rays of white dwarfs includes a variety of “Emission lineE The emission line is light (X-ray) of a particular energy emitted when electrons in the intra-atomic fell on the inside close to the nucleus more. Since the structure of the emission line determines the type and temperature of the element, the information of plasma that emits it can be obtained from the emission line. Because the universe is rich in iron, the emission lines of iron in 6 to7 keV are particularly important in X-ray astronomy. The XRT and XIS system of “SuzakuEhave a high energy resolution and excellent light-gathering power in this area. As showed by an arrow in Fig. 2, such as iron and various bright line were obtained exactly by the observation from “SuzakuE depending on the results, temperature and density distribution of the plasma were measured accurately.

The weight of the EX Hydrae white dwarf has a double difference with value determined by traditional X-rays model and the motion of the star. The Movement speed of both of the white dwarf and companion star can be measured by measurement according the motion of stars. And accomplish high accuracy can be taken by the system that “eclipseEhid behind the shadow each other are observed. It is a rare occasion as Hydra EX star meets this. In addition, it was calculated that the length of the plasma flow is very short than the radius of the white dwarfs in the traditional model, but it was also observed that it is comparable to the radius of the white dwarfs.

When data of the EX Hydrae obtained by “SuzakuEimported into our model (Fig. 3), the weight of white dwarfs and plasma flow rate per unit area was fined splendidly. The obtained weight of the white dwarf is 0.63+0.14-0.17 times the weight of the sun. It was significantly massive than 0.42±0.023 times obtained using traditional model of X-ray. On the other hand, 0.79±0.023 times that obtained from the motion of the stars, which is barely but matched in the range of error. Also, the length of the plasma flow is estimated to one third of the white dwarf radius, it was consistent with the observations above. From these, the method to capture the physics of the plasma flow in the traditional model is not enough, so it is clearing that our model represents a more realistic. In this way, we have established a high degree the measurement method of white dwarfs weight by X-ray with an accuracy than ever before. At the present time, we are working to build X-ray model incorporates the cooling of the plasma by the magnetic field, and aiming to further accuracy improvement.

Figure 3
Figure 3. The X-ray of the EX Hydrae with our model applied. (Top)
The cross points are the data, and the size of the cross indicates the error in the data. The solid line is our model. The lower half part shows the differences between the data and the model.


It is a strength of the method for measuring the weight of white dwarfs by the X-ray that it is possible to take measurement with high accuracy without a observation of such as eclipse. By using this method, regardless of the type of system, it is possible to measure the weight of many white dwarf of close binary star.

For more accurate measurements

ASTRO-H, a subsequent X-ray astronomy satellite with powerful observation equipment will be launched in 2015. The duty of ASTRO-H can measure the weight of the white dwarfs remarkably accurate.

Moreover, it might have been thought that our model is insufficient or wrong. However, we are enjoying it very much.

(Hayashi Takayuki)

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