Long-term optical spectral monitoring of a changing-look AGN NGC 3516 I:
  Continuum and broad-line flux variability
  Here we present the long-term optical spectral monitoring of a changing-look
active galactic nuclei (AGN) NGC 3516 that covers 22 years (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 UV and X-ray may indicate
that there is an obscuring region which is responsible for this.
  The line profiles are also changing. The mean profiles of the broad Halpha
and Hbeta lines show shoulder-like structure in the wings, and enhanced peak,
that may indicate a complex BLR. The rms-profiles of both lines seem to have
the same shape and width of around 4200 km/s, indicating practically the same
kinematics in the Halpha and Hbeta emitting regions.
  Measured time-lags between the continuum and Halpha and Hbeta broad-line
variability are ~15 and 17 days, respectively, that in combination with the
broad lines width allows us to estimate the NGC 3516 central black hole mass.
We find that the black hole mass is 4.73+-1.40 x 10^7M_sun which is in
agreement with previous estimates.
