THE INTENSITY AND SPECTRUM OF THE DIFFUSE-X-RAY BACKGROUND

We report the first measurement of the intensity and spectrum of the diffuse X-ray background in the 0.16-3.5 keV band which is free from contamination by sources with fluxes greater than approximately 4 x 10(-14) ergs cm-2 s-1. This result has been made possible by the development of a number of techniques for reducing cosmic-ray contamination and instrumental artifacts in the data collected by the Einstein Observatory imaging proportional counter. We describe these techniques in some detail and present an example of their utility by displaying X-ray images of the Large Magellanic Cloud both before and after their application; we also provide cautionary notes on the analysis of diffuse emission detected by this instrument without prior application of source subtraction and flat-fielding algorithms such as those presented here. Our analysis of the background data reveals a limit on the mean absolute intensity for the extragalactic emission I(x)(0.16-3.5 keV) approximately 5.6 x 10(-8) ergs cm-2 s-1 sr-1. The intensity is dependent on Galactic longitude even when only high Galactic latitude data are used, allowing us to set a lower limit on the Galactic contribution to the emission in this band. The spectrum of the total background is consistent with a power law of slope approximately 0.7 between 0.5 and 3.5 keV with a steep rise toward lower energies. The intensity we measure is greater than that expected from an extrapolation of the HEAO A-2 results at higher energies, and the slope is steeper than that which obtains in the 3-20 keV band. We briefly discuss the implications of these results for the origin of the cosmic X-ray background.