First long-term optical spectro-photometric monitoring of a binary black
  hole candidate E1821+643: I. Variability of spectral lines and continuum
  We report the results of the first long-term (1990-2014) optical
spectro-photometric monitoring of a binary black hole candidate QSO E1821+643,
a low-redshift high-luminosity radio-quiet quasar. In the monitored period the
continua and H$\gamma$ fluxes changed for around two times, while the H$\beta$
flux changed around 1.4 times. We found the periodical variations in the
photometric flux with the periods of 1200, 1850 and 4000 days, and 4500 days
periodicity in the spectroscopic variations. However, the periodicity of
4000-4500 days covers only one cycle of variation and should be confirmed with
a longer monitoring campaign. There is an indication of the period around 1300
days in the spectroscopic light curves, but with small significance level,
while the 1850 days period could not be clearly identified in the spectroscopic
light curves. The line profiles have not significantly changed, showing an
important red asymmetry and broad line peak redshifted around +1000 km
s$^{-1}$. However, H$\beta$ shows broader mean profile and has a larger
time-lag ($\tau\sim120$ days) than H$\gamma$ ($\tau\sim60$ days). We estimate
that the mass of the black hole is $\sim2.6\times10^9\rm M_\odot$. The obtained
results are discussed in the frame of the binary black hole hypothesis. To
explain the periodicity in the flux variability and high redshift of broad
lines we discuss a scenario where dense gas-rich cloudy-like structures are
orbiting around a recoiling black hole.
