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The Forefront of Space Science

The Origin of Mysterious X-ray Emission was Solar Wind !
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Sometimes we make unexpected discoveries

Suzaku is an X-ray astronomy satellite, i.e., an orbiting observatory designed to observe X-ray objects. At the mention of X-ray objects, many people may think of black holes, supernova remnants, active galactic nuclei (AGN), galaxy clusters, etc. Indeed, such themes have frequently appeared on this website. Astronomical observations are usually performed with a clear, specific goal such as “We will study x.” Nevertheless, in some cases, accidentally observed events lead to unexpected discoveries. This article describes such a case concerning X-ray emission near the earth whose origin is solar wind. This emission is not well known even in the X-ray astronomy community. With an event that Suzaku happened to observe, it was confirmed that such X-ray emission was caused by “charge exchange” between ions in the solar wind and neutral hydrogen atoms around the earth.

“Mysterious X-ray enhancement” and “charge exchange emission”

The German ROSAT satellite launched in 1990 performed an all-sky survey in soft X-rays and provided us with a precise all-sky map. From the data retrieved, researchers noticed that there were cases where the intensity of soft X-ray background radiation more than doubled in a time scale of around one day. “X-ray background radiation” describes X-rays coming from all directions in the universe. It is thought that more than half of such radiation in the soft X-ray region comes from hot interstellar matters of around 1 million Kelvin present in the disk and halo of our Galaxy. If hot interstellar matters are the source, however, such rapid changes in intensity occurring in just one day would be impossible. Astronomers presumed that the “mysterious X-ray enhancement” must be an event occurring near the earth, but were unable to identify its origin.

A clue to pinpointing the origin of the mysterious X-ray enhancement was provided by quite an unexpected finding: the discovery of X-rays from comets. In 1996, comet Hyakutake approached only 0.1 AU (astronomic unit) away from the earth and observations were conducted in various wavelengths. Since X-rays are generally emitted from hot gases of 1 million Kelvin or more, it is unthinkable that comets, ice and dust-clustered objects, alone would emit X-rays. Contrary to our expectations, strong soft X-ray emission was observed from comet Hyakutake. Intrigued by this discovery, X-ray emission from comets has been confirmed in succession, which convinced us that soft X-ray emission is a common phenomenon with many comets. With this new finding, theoretical and experimental research on soft X-ray emission mechanism advanced. As a result, it was posited that “soft X-rays from comets are emitted by the charge exchange interaction between ions in the solar wind and neutral materials in the comet.”

Charge exchange is a phenomenon wherein electrons move from one particle to another when two particles (e.g., atoms, molecules, and ions) collide with each other. The colliding particles are highly ionized ions (e.g., almost fully ionized carbon and oxygen ions) within the solar wind while targeted particles are atoms or molecules existing around comets or the earth. Electrons exiting from targeted particles enter the outer orbit of a colliding particle (i.e., ion) and finally fall into the innermost orbit, emitting light with a specific wavelength (Fig. 1).

Figure 1
Figure 1. Conceptual illustration showing an electron moving from a neutral atom to an ion by charge exchange interaction and emitting a specific energy light (emission line). In this case, an electron moves from a hydrogen atom to the 4th orbit of a fully ionized carbon ion and then transits to the innermost orbit while emitting X-ray.

As it was realized that X-ray emission from comets was caused by the charge exchange process, researchers began to think that part of the soft X-ray background radiation was caused by the charge exchange interaction between solar wind and neutral materials existing around the earth and in the heliosphere. In addition, it was proved that the mysterious X-ray enhancement discovered by the ROSAT all-sky survey was correlated with a flux of solar wind (protons). Thus, it was presumed that such enhancement must also be caused by the charge exchange interaction between solar wind and neutral hydrogen around the earth.

As stated above, when a charge exchange occurs, photons with specific energy, or emission lines, are emitted from the highly ionized ions. Researchers could not identify such emission lines, however, because of inadequate spectral resolution of ROSAT. Recently, the Chandra and XMM-Newton satellites observed emission lines probably caused by charge exchange interaction. However these satellites’ spectral resolution for soft X-ray is not still sufficient and, therefore, far better spectral data were demanded.

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