A High Spectral Resolution Observation of the Soft X-Ray Diffuse Background with Thermal Detectors

A high spectral resolution observation of the diffuse X-ray background in the 60-1000 eV energy range has been made using an array of 36 1 mm² microcalorimeters flown on a sounding rocket. Detector energy resolution ranged from 5 to 12 eV FWHM, and a composite spectrum of ~1 sr of the background centered at l = 90°, b = +60° was obtained with a net resolution of ~9 eV. The target area includes bright keV regions but avoids Loop I and the North Polar Spur. Lines of C VI, O VII, and O VIII are clearly detected with intensities of 5.4 ± 2.3, 4.8 ± 0.8, and 1.6 ± 0.4 photons cm^-2 s^-1 sr^-1, respectively. The oxygen lines alone account for a majority of the diffuse background observed in the ROSAT R4 band that is not due to resolved extragalactic discrete sources. We also have a positive detection of the Fe-M line complex near 70 eV at an intensity consistent with previous upper limits that indicate substantial gas-phase depletion of iron. We include a detailed description of the instrument and its detectors.