Solar Wind Charge Exchange Background

A component that can be very problematic originates in the solar system and in the near-Earth environment. SWCX occurs when highly ionized ions in the solar wind collide with either exospheric hydrogen near the Earth or the neutral interstellar medium passing through the solar system and pick up an electron. The acquired electron is typically in a highly excited state which then radiatively decays.

Prominent SWCX emission lines include those from C VI, O VII, O VIII, Ne IX, and Mg XI that are some of the same lines that are important diagnostics of astrophysical plasmas. If an observation is long enough, some evidence for SWCX contamination can possibly be found by comparing the $0.5-0.7$ keV and $2.0-8.0$ keV light curves (see Carter & Sembay 2008). If the lower-energy light curve varies while the high-energy light curve doesn't, the data may be affected. Another SWCX diagnostic is the presence of very strong O VIII and Mg XI lines in the cosmic diffuse background component. Additional observations of a region may be the only way to determine the presence and minimum extent of SWCX contamination. Further discussion of solar wind charge exchange can be found in Kuntz (2019).

To model the SWCX component, Gaussian lines at the characteristic energies (see Table 7 for most common lines in the EPIC energy range) with zero width are added to the spectral fit. In the fit, the line energies should be fixed until a reasonable fit is obtained and then allowed to float.

Generally, for imaging analysis, SWCX is not an issue. The SWCX lines typically have strengths that are small enough that they can be accommodated by the LHB/Galactic halo components, thus producing incorrect values for the temperatures and normalizations of those spectral components.