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

Abstract

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 Hα and Hβ 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 Hα and Hβ emitting regions. Measured time-lags between the continuum and Hα and Hβ broad-line variability are ∼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) × 107 Mo which is in agreement with previous estimates.