The discovery of an O VII emission line in the ASCA spectrum of the Seyfert galaxy NGC 3783

We report the first observation of an O VII 0.57 keV emission line in a Seyfert 1 galaxy. NGC 3783 was observed by ASCA twice over a period of 4 days in 1993 December. The source exhibited a ∼30% change in intensity between the two observations, with most of the variability taking place as a result of steepening of the continuum ≲1 keV. Spectra from both observations show intense absorption features in the 0.5-1.5 keV band, which can be well fitted by an ionized absorber model of solar composition, column density of 10^22.2 cm^-2 and ionization parameter of ∞7-8; the strongest absorption features being due to O VII and O VII. Two emission features are also seen in the spectra which we identify as O VII 0.57 keV (equivalent width ≃36 eV) and O VIII 0.65 keV (equivalent width ≃11 eV). We find these features are at the intensity predicted by the above ionized absorber model when the covering factor is close to unity. We also show that the 3-6 keV continuum of the source is well fitted by a Γ = 1.3-1.4 power-law continuum, a narrow neutral iron K-shell fluorescence line and a strong iron K-shell absorption edge, possibly corresponding to highly ionized iron. The intensity of neither of these K-shell features can be explained by the ionized absorber, and thus must be from a separate component.