We analyse the ROSAT PSPC hardness ratio and the 0.5-2 keV to 2-10 keV flux ratio of 65 Active Galactic Nuclei ( AGN ) for which there are both ROSAT archival observations available and 2-10 keV fluxes , mostly from the HEAO-1 MC-LASS survey .
We conclude that the simplest spectral model for the AGN that can 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 body both partially absorbed .
In our sample , type 1 AGN require an absorbing column around 10 ^ { 22 } { cm } ^ { -2 } with covering fractions between 20 and 100 % , while type 2 AGN display larger columns and \sim 100 \% coverage .
This simple model also provides a good link between soft and hard AGN X-ray luminosity functions and source counts .
We also consider a warm absorber as an alternative model to partial covering and find that the the presence of gas in two phases ( ionized and neutral ) is required .