The spectrum of the extragalactic X-ray background below 3 keV

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Abstract
We present the results of a ROSAT XRT/PSPC observation of a field near the North Galactic Pole containing the interacting galaxy pair NGC 4725/4747. A tidal H I plume emanating from NGC 4747 and also the extended H I disc in NGC 4725 represent ‘foreground’ screens which will, in principle, cast shadows on any extra-galactic component of the soft X-ray background. The failure to detect such shadowing in the ROSAT observation leads to a 95 per cent upper limit on the intensity of the unresolved extragalactic background of |$26.5\enspace \text {keV}\enspace \text{cm}^{-2}\enspace \text s^{-1}\enspace \text {sr}^{-1} \enspace \text {keV}^{-1}$| at 0.25 keV, after excluding discrete sources brighter than |$1.7 \times\ 10^{-14}\enspace \text{erg cm}^{-2}\enspace \text s^{-1}$| in the |$0.5-2$| keV band. This upper limit increases by a factor of approximately ∼ 1.5 if we allow for possible additional line-of-sight absorption associated with a partially ionized component of the high-latitude interstellar medium. These measurements, together with constraints at ∼ 1 keV derived from an analysis of the PSPC background spectrum, demonstrate that the extragalactic background between 0.2 and 1 keV has an energy index |$\alpha \lesssim 0.7$| (or |$\alpha \lesssim 1.0$| if the more conservative 0.25-keV upper limit applies). We briefly discuss the implications of these results for the source populations that may give rise to the X-ray background radiation.

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