2021MNRAS.508.2937M

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 amplitude of ∼0.2 mag, superimposed on a gradual dimming at a rate of ∼0.55 mag per 100 yr. 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.