The BeppoSAX broad-band spectrum and variability of the Seyfert 1 NGC 3783

BeppoSAX observed the Seyfert 1 galaxy NGC 3783 for ~5 days from 1998 June 6 to 1998 June 10. The average flux during the observation was $F_{2-10\,\rm keV}\sim 6\times 10^{-11} \;\rm erg\ cm^{-2}\ s^{-1}$. The long exposure provided a very high quality spectrum that exhibits all the typical features of Seyfert 1s. The high energy cut-off of the intrinsic continuum, at $E_{\rm c}=340\pm^{560}_{107}$ keV, was detected for the first time in this object. During the observation the source showed modest flux variations (~20% in 2–10 keV) and spectral variability that can be completely explained by a change of the intrinsic slope $\Delta \Gamma\sim 0.1$. NGC 3783 is one of the few Seyfert 1s that exhibits a soft emission component in the BeppoSAX spectrum. This soft excess is well reproduced by a black body component with temperature $kT\sim0.2$ keV. A warm absorber is also required to fit the soft X-ray spectrum. Both reprocessing features, the Compton reflection hump and the Fe emission line, were detected in our observation. In addition to a narrow iron line (as observed by Chandra) a broad component with $\sigma=0.72\pm^{1.28}_{0.27}$ keV and $EW=115\pm76$ eV, is required to fit the line. The observed Compton hump and the Fe line are well reproduced with the ionized ($\log\xi=2.73\pm^{0.04}_{0.05}$) disc reflection model of Ross & Fabian. In this case the soft excess observed below 2 keV can be accounted for by the disc emissivity and no additional soft X-rays component is required to fit the spectrum