Diffuse Soft X-ray Background
The solar system resides in the Local Hot Bubble (LHB) with a size scale of 100 pc, which can produces bright diffuse soft X-ray emission. Although LHB is irregular in shape, it is thought to have a uniform temperature of ∼0.1 keV. Its intensity, unattenuated by the Galactic absorption, varies across the sky. Its elemental abundances are largely unknown. Some fractions of O, Mg, Si, and Fe might be locked up in dust, but the exact dust depletion fraction is poorly constrained.
Within the solar system, the solar wind charge exchange (SWCX) process can also produce time-variable diffuse soft X-ray emission. The hot solar wind can pick up electrons from the neutral medium in the solar system, capturing them into excited energy levels rarely populated in other mechanisms (e.g., collisional ionization equilibrium, photoionization equilibrium). The unstable excited energy level is de-populated by line emissions. The characteristic features of SWCX are the enhanced forbidden line of the He-like triplet with respect to the resonance line and the enhanced high-order Lyman series line with respect to the Lyα line. High-resolution X-ray spectroscopy is the key to quantifying the role of SWCX in such kind of X-ray foreground emission.
