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

Abstract

The X-ray background (XRB) from 0.1 to 7 keV has been studied using high spectral and spatial resolution data from the ASCA SIS and ROSAT PSPC. 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. The spectrum of the XRB can be fit well by a single power-law from 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. 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 spectrum (with photon index of about 3); the accumulated spectrum of other non-stellar sources resolved by ROSAT also has a steeper-than-average spectrum. Fitting the XRB 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 they 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 $10^{-15}$ \ergpercmpers (in the 0.5--2 keV band). Constrained by our spectral fitting results, the major contributor of the XRB must be a single population with similar flat spectra.