Spectroscopic Separation of Solar Wind Charge Exchange, Local Bubble, and Nearby Supernova Remnant X-rays: Diffuse X-ray Spectrometer Recent Results

The Diffuse X-ray Spectrometer (DXS) was a Space Shuttle Payload of Opportunity that flew in 1993. DXS measured the spectrum of the diffuse X-ray background (DXRB) between 150 eV and 284 eV (the 1/4 keV band) using a Bragg crystal spectrometer. Higher order Bragg reflections included the OVII and OVIII features. The counting statistics and spectroscopic resolving power of the DXS measurements have yet to be rivaled in the 1/4 keV band. DXS had a 15°x15° FOV that was repeatedly scanned over a 140° arc in the Galactic plane centered roughly toward the Galactic anti-center. The Vela-Puppis and the Monogem ring supernova remnants were studied, as well 3 adjacent regions typical of the DXRB. During the 5-day Shuttle flight, the total sky-looking DXS count rate unexpectedly dropped by 20%, suggesting a significant and variable local source of X-rays, likely generated by the solar wind charge exchange mechanism (SWCX) in the geocorona and/or a passing coronal mass ejection. We use this unique dataset to: (1) Show that a state-of-the-art heliospheric SWCX model compares reasonably well to the DXS DXRB spectrum in the 190-284 eV range, but falls short in the 150-190 eV range. (2) Spectroscopically resolve the OVII forbidden and resonance lines, showing that the resonance line is somewhat stronger. This confirms there is a contribution to the DXRB from a source other than the SWCX. (3) Present spectra of the Vela-Puppis and Monogem regions cleaned of all foreground X-ray emission and compare to standard collisional ionization equilibrium plasma models. The discrepancies between the models and data highlight the need for continued progress in understanding the L-shell ions of Mg, Si, S and the M-shell ions of Fe. (4) Present the first isolated spectrum of the SWCX in the 1/4 keV band that resolves lines/line complexes.