THE SOFT-X-RAY DIFFUSE BACKGROUND OBSERVED WITH THE HEAO 1 LOW-ENERGY DETECTORS

We present the conclusions of our study of the diffuse soft X-ray background as surveyed by the low-energy detectors (LED) of the A-2 experiment aboard the HEAO 1 satellite. The observed sky intensities are presented as maps of the diffuse X-ray background sky in several energy bands covering the energy range 0.15-2.8 keV. High-quality spectra of the soft X-ray diffuse background (SXDB) were obtained for regions near the north ecliptic pole (NEP) and the south ecliptic pole (SEP). From the spectral analysis we find that the SXDB between 1.5 and 2.8 keV, assuming a power law form with photon number index alpha = 1.4, has a normalization constant of A = 10.5 +/- 1.0 photons cm-2 s-1 sr-1 keV-1. Below 1.5 keV the spectrum of the SXDB exceeds the extrapolation of this power law. The low-energy excess for the NEP can be fitted with emission from a two-temperature equilibrium plasma model with the temperatures given by log T1 = 6.16 and log T2 = 6.33. The first component is unabsorbed and has an emission measure EM = 2.5 x 10(-3) cm-6 pc and the second component has EM = 5.3 x 10(-3) cm -6 pc with an absorption hydrogen column density of 3.6 x 10(20) cm-2 . As an astrophysical application of these data, we fitted the NEP spectrum with a hydrostatic model of Bloemen. We find that this model is able to account for the spectrum below 1 keV, but fails to yield the observed Galactic latitude variation.