Lw. Chen et al., ASCA AND ROSAT OBSERVATIONS OF THE QSF3-FIELD - THE X-RAY-BACKGROUND IN THE 0.1-7-KEV BAND, Monthly Notices of the Royal Astronomical Society, 285(3), 1997, pp. 449-471
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. Anal
ysing both the diffuse background radiation and resolved sources, we h
ave 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 ke
V; to account for the Galactic emission below 1 keV, a model with a po
wer law plus two thermal components fits well to the measurements of A
SCA and ROSAT from 0.1 to 7 keV. Overall, the photon index of the powe
r-law model ranges from 1.4 to 1.5, and no obvious excess is found bet
ween 1 and 3 keV as predicted from some previous observations. Below 1
keV, the models become more complicated and involve a mixture of extr
agalactic and Galactic sources. As some of the extragalactic contribut
ions should be from point sources, we have examined the ASCA and ROSAT
spectra of resolved sources individually: a stellar source having a w
ell-fitted thermal spectrum and two AGN having a much steeper power-la
w spectrum (with a photon index of about 3); the accumulated spectrum
of other nonstellar 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 contribut
e 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 tha
n a few times 10(-15) erg cm(-2) s(-1) (in the 0.5-2 keV band). Constr
ained by our spectral fitting results, the major contributor of the X-
ray background must be a single population with similar flat spectra.