ASCA and ROSAT observations of the QSF3 field: the X-ray background in the 0.1-7 keV band

The X-ray background (XRB) from 0.1 to 10 keV has been studied using data observed with the ASCA SIS and ROSAT PSPC. We have carried out a series of joint spectral fitting of the ASCA and the ROSAT data, 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 by 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-fit thermal spectrum and two AGNs having a much steeper power-law spectra; the accumulated spectrum of other non-stellar sources resolved by ROSAT also has a steeper-than-average spectrum. This provides evidence that none of the resolved sources has a spectrum hard enough to match the XRB in a broad band. Fitting the XRB spectrum observed by ASCA and the accumulated point source spectrum by ROSAT together by varying the contribution from resolved point sources to the background, we find that the contribution is less than 30 per cent in the 0.5--2~keV band and drops to several per cent in 2--10~keV. This fraction is provided by sources brighter than a few times 10^-15~\ergpercmpers (in the 0.5--2~keV band)