Evidence for periodicity in 43 year-long monitoring of NGC 5548
  We present an analysis of 43 years (1972 to 2015) of spectroscopic
observations of the Seyfert 1 galaxy NGC 5548. This includes 12 years of new
unpublished observations (2003 to 2015). We compiled about 1600 H$\beta$
spectra and analyzed the long-term spectral variations of the 5100 \AA\
continuum and the H$\beta$ line. Our analysis is based on standard procedures
including the Lomb-Scargle method, which is known to be rather limited to such
heterogeneous data sets, and new method developed specifically for this project
that is more robust and reveals a $\sim$5700 day periodicity in the continuum
light curve, the H$\beta$ light curve and the radial velocity curve of the red
wing of the H$\beta$ line. The data are consistent with orbital motion inside
the broad emission line region of the source. We discuss several possible
mechanisms that can explain this periodicity, including orbiting dusty and
dust-free clouds, a binary black hole system, tidal disruption events, and the
effect of an orbiting star periodically passing through an accretion disc.
