1994A&A...291..420M

Result. are presented from a ROSAT observation of the Seyfert Galaxy Mkn 766. The source displays large amplitude variations over timescales of a few days. A detailed analysis of the flux and spectral variability is presented. We show that the soft (0.1 to 0.9keV) and hard (0.9 to 2.0keV) part of the spectrum display different behaviour: the soft part hardens as the source brightens while the hard part does not change significantly. This result rests on a hardness ratio analysis and is therefore model independent. Spectral analysis shows that variations cannot be explained by variations in the optical depth of an absorption edge. Variations in the optical depth or ionization state of a warm absorber also appear to be an unlikely explanation. Emission from an accretion disk can explain the observed spectrum, the spectral variations and the flux variability of Mkn 766.