| Abstract
| - We present the broad band analysis of two BeppoSAX observations of the Seyfert 1 Mkn 509. In 2000 the source was in a typical flux state, $F_{2-10~\rm keV}=2.7\times 10^{-11}~ \;\rm erg~cm^{-2}\,s^{-1}$, while in 1998 it was found in a high flux state, $F_{2-10~\rm keV}=5.7\times 10^{-11}~ \;\rm erg~cm^{-2}\,s^{-1}$. A comparison between the two states shows a energy-dependent flux variation of about a factor of three and a factor of two in the LECS (0.15-3 keV) and MECS (1.5-10 keV), respectively, while in the PDS (13-200 keV) the difference is marginal. A soft excess, a narrow iron line and a Compton reflection hump above 10 keV are clearly apparent in the residuals after fitting the spectra with a simple power law. We tested two alternative models. In the first the iron line and the high energy bump are well reproduced by reprocessing in a cold and Compton thick material. The intensity of the iron line (also consistent with a Chandra measurement) as well as the normalization of the reflection hump are consistent with a constant in the two epochs: this, combined with the fact that the line is narrow as observed by Chandra, suggests a common origin from distant and optically thick matter. This model further requires a component to model the soft excess: the empirical choice of two black bodies accounts well for the excess in both observations; their combined strength was a factor of about three higher in the high than in the low flux state defined above. However, the relative contribution of the soft excess is higher in the low flux state. In the second model we attempted to reproduce all spectral features, except for the narrow cold line, with reflection from an ionized disc. This model is successful only in the high flux state, but it fails in the low flux state, when the soft excess is only partially accounted for. In either model, the slope of the power law is greater in the high than in the low flux state, ( $\Delta\Gamma \sim0.2$), in agreement with a behaviour known to be shared by several objects of the same type.
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