Periodic variability of the z=2.0 quasar QSO B1312+7837
  We report here the first results from a 15-yr long variability monitoring of
the z=2.0 quasar QSO B1312+7837. It shows luminosity changes with a period
P~6.13 yr (P~2.04 yr at rest frame) and an amplitude of ~0.2mag, superimposed
on a gradual dimming at a rate of ~0.55mag per 100 yrs. Two false periods
associated with power peaks in the data windowing function were discarded. The
measured period is confirmed with a bootstrapping Monte-Carlo simulation. A
damped random walk model yields a better fit to the data than a sine-function
model, but at the cost of employing some high frequency variations which are
typically not seen in quasars. We consider the possible mechanisms driving this
variability, and conclude that orbital motion of two supermassive black holes -
result from a recent galaxy merger - is a possible explanation.
