The X-ray background from 0.1 to 7 keV has been studied using data of high spectral and spatial resolution from the ASCA Solid-state Imaging Spectrometers and ROSAT Position Sensitive Proportional Counter. Analysing both the diffuse background radiation and resolved sources, we have carried out a series of joint spectral fits of the ASCA and ROSAT data. As found previously with ASCA data alone, the spectrum of the X-ray background can be fitted well by a single power law from 1 to 7 keV; to account for the Galactic emission below 1 keV, a model with a power law plus two thermal components fits well to the measurements of ASCA and ROSAT from 0.1 to 7 keV. Overall, the photon index of the power-law model ranges from 1.4 to 1.5, and no obvious excess is found between 1 and 3 keV as predicted from some previous observations. Below 1 keV, the models become more complicated and involve a mixture of extragalactic and Galactic sources. As some of the extragalactic contributions should be from point sources, we have examined the ASCA and ROSAT spectra of resolved sources individually: a stellar source having a well-fitted thermal spectrum and two AGN having a much steeper power-law spectrum (with a photon index of about 3); the accumulated spectrum of other non-stellar sources resolved by ROSAT is also steeper than the average AGN spectrum. Fitting the X-ray background spectrum observed by ASCA and the accumulated point source spectrum by ROSAT together by varying the contribution from steep-spectrum sources, such as quasars, to the background, we find that the steep-spectrum sources contribute less than 30 per cent in the 0.5–2 keV band and drop to below 10 per cent over 2–10 keV. This fraction is provided by sources brighter than a few times 10−15 erg cm−2 s−1 (in the 0.5–2 keV band). Constrained by our spectral fitting results, the major contributor of the X-ray background must be a single population with similar flat spectra.
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Chen et al. (1997) studied this question.