The spectrum of the extragalactic X-ray background below 3 keV.
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 disk in NGC 4725 represent `foreground' screens which will, in principle, cast shadows on any extragalactic 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 keV cm^-2 s^-1^ sr^-1^ keV^- 1^ at 0.25 keV, after excluding discrete sources brighter than 1.7 X 10^-14^- erg cm^-2^ s^-1^ in the 0.5-2 keV band. This upper limit increases by a factor of ~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 α <~ 0.7 (or α <~ 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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- March 1994
- DOI:
- 10.1093/mnras/267.2.270
- Bibcode:
- 1994MNRAS.267..270B
- Keywords:
-
- Absorption Spectra;
- H I Regions;
- Interacting Galaxies;
- Interstellar Gas;
- Interstellar Matter;
- X Ray Sources;
- Rosat Mission;
- Tides;
- X Ray Astronomy;
- Astrophysics