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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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Shining white dwarfs by the X-rays

Single white dwarfs shines by residual heat of stellar, and became cool and dark gradually, then finally disappears. However, when other star join close binary, the situation is reversed. The white dwarfs have high density, it’s surface gravity are very strong. Accordingly, as the gas from the partner of close binary stars (companion star) fall onto the white dwarf, a huge amount of energy to convert temperature to several hundreds of million degrees can be got. It will become the energy source shining white dwarfs.

When the gas falls on a white dwarf, in cases where the magnetic field of white dwarf is strong (more than 0.1 million Gauss. It is 0.2 million times of geomagnetism), the gas is captured by magnetic field (Fig.1). The gas caught in magnetic field falls freely along the lines of magnetic force. Its speed is reached thousands of kilometers per second.

Figure 1
Figure 1. Imaginary picture of the close binary star of a white dwarf with strong magnetic fields.
Gas from the companion star is caught by magnetic field, and falls to the white dwarf along the magnetic field lines.


The fall velocity is beyond the velocity of sound in gas, so the gas flows causes a “Shock WaveEin the near surface of white dwarfs. Shock wave is a wave around the mass movement in supersonic with pressure and temperature can be changed discontinuously. When the speed of movement of the material is fast enough compared with the sound, three quarters of the kinetic energy of the shock wave in supersonic gas flows are converted into the heat energy. When this is applied to a gas falling into a white dwarf, the gas heated to several hundreds of million degrees, and the atom becomes plasma by ionizing.

Material is luminous in the electromagnetic wave of the energy?wavelength?corresponding to the temperature. The human body temperature is about 40 degrees, so the infrared rays are transmitted. Solar surface sends visible light with about 6000 degrees, and a plasma with a few hundreds of million degrees is luminous in X-ray. Plasma be produced by the shock wave will fall in white dwarf, accompanied by a cool in radiating X-ray. Conversely, depending the energy of electromagnetic waves, to measure the temperature of an object that emits it.

As mentioned previously, the source of the heat energy while fall on a white dwarfs is the gravity of the white dwarf. As the stronger the gravity, plasma falling on a heavier white dwarfs becomes a higher temperature. Thus, the temperature of the plasma falls on white dwarfs can be measured by X-rays, and the weight of the white dwarfs can be estimated in a measured result.

The method for measuring weight of the white dwarfs by X-rays

After being generated by the shock wave, the plasma of white dwarfs along a strong magnetic field falls on the white dwarf. We have created a detailed model of X-ray spectrum of the intensity distribution of the X-ray energy from the plasma flow. In addition to the difference in the weight of the white dwarfs that had been taken from the previous, the shape of the plasma flow and the difference of the flow rate are newly fetched.

The shape of the plasma flow is assumed to be a circular cylinder in the model so far. But in fact, it was thought the closer the white dwarfs, the thinner the shape like a trumpet along the lines of magnetic force. In this type of flow, the action flow is faster instead of temperature drops. This action, in common with the flow of the river, becomes faster at narrowly width of the river. Due to the amount of gas from a companion star with a difference in each system, the flow rate per unit area of plasma flow into a white dwarfs determines the fact of the plasma cools down. Also, weight of white dwarfs governs the strength of gravity.

Based on conditions close to reality like this, the distribution of the density and temperature of the plasma stream can be obtained by solving the system of equations of the equations of motion, the law of conservation of energy, the law of conservation of mass and equation of state (Relationship of pressure and density) in the laws of physics. The obtained temperature and density distribution can be used to compute the X-ray spectrum from the whole plasma flow, and to create a model of X-ray spectrum of the white dwarfs with a strong magnetic field. The weight of the white dwarfs and the flow rate per unit area of plasma flow can be measured by applying the model to observed data.

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