Long-term optical spectral monitoring of a changing-look active galactic nucleus NGC 3516-I. Continuum and broad-line flux variability

In this paper, we present the long-term optical spectral monitoring of a changing-look active galactic nucleus (AGN), NGC 3516, covering 22 yr (from 1996 to 2018). We explore a variability in the broad lines and continuum, finding that the continuum is changing by more than a factor of 2, while the broad lines are varying by more than a factor of 10. The minimum of activity is observed in 2014, when the broad lines almost disappeared. We confirm that NGC 3516 is a changing-look AGN, and the absorption seen in the ultraviolet and X-ray may indicate that there is an obscuring region that is responsible for this. The line profiles are also changing. The mean profiles of the broad Ha and H beta lines show a shoulder-like structure in the wings, and an enhanced peak, which could indicate a complex broad-line region. The rms profiles of both lines seem to have the same shape and width, of around 4200 km s(-1), indicating practically the same kinematics in the Ha and H beta emitting regions. Measured time-lags between the continuum and Ha and H beta broad-line variability are similar to 15 and 17 d, respectively. In combination with the broad-line width, these measurements allow us to estimate the central black hole mass of NGC 3516. We find that the black hole mass is (4.73 +/- 1.40) x 10(7) M-circle dot which is in agreement with previous estimates.