Soft versus hard X-ray emission in active galactic nuclei: partialcovering and warm-plus-cold absorber models

We analyse the ROSAT PSPC hardness ratio and the 0.5-2 keV to 2-10 keV fluxratio of 65 Active Galactic Nuclei (AGN) for which there are both ROSATarchival observations available and 2-10 keV fluxes, mostly from the HEAO-1MC-LASS survey. We conclude that the simplest spectral model for the AGN thatcan accommodate the variety of X-ray colours obtained is a standard power law(with energy spectral index $\alpha\sim 0.9$) plus a $\sim 0.1$ keV black bodyboth partially absorbed. In our sample, type 1 AGN require an absorbing columnaround $10^{22}\, {\rm cm}^{-2}$ with covering fractions between 20 and 100\%,while type 2 AGN display larger columns and $\sim 100\%$ coverage. This simplemodel also provides a good link between soft and hard AGN X-ray luminosityfunctions and source counts. We also consider a warm absorber as an alternativemodel to partial covering and find that the the presence of gas in two phases(ionized and neutral) is required.Comment: 10 pages, Latex (mn.sty), 1 table, 5 figures included (epsf), postscript version also available via anonymous ftp at ftp://astsun1.unican.es/pub/ceballos/ . Accepted for publication in MNRA