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 mm(2) microcalorimeters own on a sounding rocket. Detector energy resolution ranged from 5 to 12 eV FWHM, and a composite spectrum of similar to1 sr of the background centered at l = 90degrees, b = +60degrees was obtained with a net resolution of similar to9 eV. The target area includes bright 1/4 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.