Science
Scientific Objectives
The main goal of DIXE is to probe the physical properties (e.g., temperature, density, elemental abundances, kinematics) of the Galactic hot baryons. Galactic hot baryons are in fact the foreground of any extragalactic X-ray observations. This adds to the scientific return of DIXE.
01
Large-scale Hot Structures
The origin of the eROSITA bubble is still debated. Some argue it originates from the past activity of Milky Way’s SMBH (Sgr A⋆), while others favor a stellar feedback origin. The confusion mainly arises from the relatively poor measurement of elemental abundances as well as the emission measure distribution as a function of temperature. In addition, DIXE can constrain the kinematics if the bubbles have a significant Doppler shift due to their motion. DIXE observation can easily verify theoretical predictions. Another example is the Cygnus loop, which is a nearby supernova remnant (SNR) where charge exchange features and the “low-abundance problem” were suggested for some local structures. A global high-resolution X-ray spectroscopic view of the entire SNR is still lacking.
02
The Milky Way Halo
The Milky Way Halo (MWH) is postulated as a spherical symmetric halo to the first-order approximation. At least some (if not all) fractions of MWH have a temperature of ∼ 0.1 − 0.2 keV. Similar to the LHB, the metallicity of the MWH is largely unknown due to the lack of high-resolution X-ray spectra. Moreover, a hotter component with a temperature of ∼0.7 keV accompanied by a super-solar Ne/O abundance ratio has also been suggested. Such a component is not expected within the framework of a single-temperature hot Galactic halo. If this component is verified with DIXE observations, we will further investigate if it originates from M dwarf stars in the Galactic disk or if it is part of a two-temperature Galactic halo.
03
The Local Hot Bubble
The solar system resides in the Local Hot Bubble (LHB) with a size scale of 100 pc, which also produces bright 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.
04
Solar Wind Charge Exchange
Within the solar system, the solar wind charge exchange (SWCX) process can produce time-variable X-ray foreground 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.
