Short-timescale X-ray spectral variability in the Seyfert 1 galaxy NGC 3783

Aims. We report on the X-ray time-resolved spectral analysis of XMM-Newton observations of NGC 3783. Our main goal is to detect transient features in the Fe K line complex in order to study the dynamics of the innermost accretion flow. Methods. We reanalyse archival observations of NGC 3783, a bright local active galactic nucleus, for which a transient Fe line was reported, complementing this data set with new available observations. This results in a long set of observations which can allow us to better assess the significance of transient features and possibly test their recurrence time. Moreover, as the new data catch the source in an obscured state, this analysis allows also to test whether the appearance and disappearance of transient features is linked to the presence of obscuring gas. Results. We detect discrete features at the ¥90% significance level both in emission and in absorption at different times of the observations, split into 5 ks time-resolved spectra. The overall significance of individual features is higher in the obscured dataset. The energy distribution of the detections changes between the two states of the source, and the features appear to cluster at different energies. Counting the occurrences of emission and absorption lines at the same energies, we identify several groups of ¥3Ïdetections: emission features in the 4- 6 keV band are present in all observations and are most likely due to effects of the absorber present in the source; an emission line blend of neutral Fe Kβ and ionised Fe Kα is present in the unobscured dataset; absorption lines produced by gas at different outflowing velocities and ionisation states show an increase in energy between the two epochs, shifting from 6.6 keV to 6.7-6.9 keV. The representation of the features in a time- energy plane via residual maps highlights a possible modulation of the Fe Kα line intensity linked to the clumpiness of the absorbing medium.