>Soft X-ray Background and Solar Wind Charge Exchange
Soft X-ray Background and Solar Wind Charge Exchange
The soft X-ray diffuse background (SXRB) is the low-energy (~0.1 to 1.5 keV) emission that is not due to discrete objects such as X-ray binaries, supernova remnants, or planetary nebulae. The emission is due to diffuse hot gas with temperatures of a few million degrees. There are multiple sources of this gas, from the nearby interstellar medium (ISM), to the hot corona (or halo) thought to surround the Galaxy, to the vast filaments of plasma that form the "cosmic web" at cosmological distances. It is well known that hot ISM in the disk of the Galaxy is produced primarily by supernovae, but the quantity of that gas is very poorly understood. The emission from the million degree (1MK) gas is also very strongly absorbed by the metals in cool gas, meaning that we learn about the 1MK gas if it is nearby, while emission from even hotter gas, which is less strongly absorbed, is observed to much greater distances. Only by determining the distance to the X-ray emitting gas can we determine its mass and other properties. Ironically, the primary method by which to determine the distance to the hot gas requires that emission to be absorbed by closer cool gas whose distance we know through other methods. Similarly, due to uncertainty in the distance to the emitting gas, the mass of the Galactic halo is uncertain by a factor of ten; if the mass is as high as some studies suggest, the X-ray emitting halo may solve the "missing baryon problem". Finally, the emission due to the ISM and the halo is similar to that expected from the "cosmic web", which makes measuring the hot gas at cosmological distances more difficult.
Charge exchange occurs when an ion interacts with a neutral atom and an electron is transferred from the neutral atom to the ion. That electron enters an excited state and X-rays are emitted when the electron moves to the ground state. Charge exchange occurs any place that highly ionized atoms (such as O^+7) can interact with cool neutral gas, such as the places where supernova remnants and stellar winds collide with molecular clouds. Solar wind charge exchange (SWCX) is due to the highly ionized solar wind interacting with the outermost part of the Earth's atmosphere. There, it produces highly time-variable X-ray emission through which most X-ray telescopes observe. Separating the SWCX emission from the emission due to more distant objects is quite difficult, especially as the spectrum of the SWCX emission is very poorly understood. Improved understanding of SWCX emission will allow better removal of that emission from observations of celestial sources of X-rays. However, it will also lead to better understanding of the solar wind and the Earth's magnetosheath.
