Long-term optical spectral monitoring of a changing-look active galactic nucleus NGC 3516

Context. We analyze the broad H β line profile variability of a “changing look” active galactic nucleus (CL-AGN) NGC 3516 over an extensive period of 25 years (from 1996 to 2021). The observed change in the broad line profile may indicate a change in the geometry of the broad line region (BLR). The main objective is to follow and understand the change in the BLR over a long period as well as its connection to the CL mechanism. Aims. Using spectral line profiles, we aim to explore changes in the kinematics and dimensions of the BLR in NGC 3516. We consider two possible scenarios: the changes in the broad-line emission are either caused by a decrease of ionisation continuum emission or by the BLR obscuration by outer dusty regions. With this investigation, we aim to clarify the CL mechanism of this AGN. Methods. We analyzed the spectral band around the H β line as well as the broad H β line parameters and how they change over time. We modelled the broad-line profiles, assuming that there is an emission from the accretion disc superposed with emission from a surrounding region that is outside the disc. Results. We find that in the type 1 activity phase occurring when the strong broad emission lines are observed, the BLR is very complex. There is a clear disc-like BLR that contributes to the broad line wings and an additional intermediate line region (ILR) that contributes to the line core. In the high-activity phase, the ILR emission is close to the center of the line, although in some cases, it is slightly shifted to the red. In the low-activity phase (i.e. type 2 phase), the ILR component has a significant shift to the blue, indicating an outflow. Conclusions. We propose that the changing-look mechanism in NGC 3516 is rather connected with the intrinsic effects than with an outer obscuring region. It may still be possible that the dust plays an important role in the low-activity phase when it is coming from within the BLR, leading to a dusty BLR. In this way, it would cause a decrease in the ionisation and recombination rates.