The possible long-term periodic variability of the extremely luminous quasar WISE J090924.01+000211.1

The extremely luminous infrared galaxy (ELIRG), WISE J090924.01$+$000211.1 (hereafter WISE J0909$+$0002, $z=1.87$) is an extraordinary object with a quasar aspect. This study performed monitoring observations of WISE J0909$+$0002 with the 105$\:$cm Murikabushi telescope, Okayama and Akeno 50$\:$cm telescopes/MITSuME ($g^{\,\prime}$, $R_{\rm c}$, and $I_{\rm c}$ bands), and the SaCRA 55$\:$cm telescope/MuSaSHI (r, i, and z bands) for three years. We obtain the following results by combining the UV/optical light curves of the CRTS, Pan-STARRS, and ZTF archive data, and our observational data: (1) the light curves of WISE J0909$+$0002 present quasi-periodic (sinusoidal) oscillations with the rest-frame period of $\sim \!660$–$689\:$d; (2) the structure functions of WISE J0909$+$0002 do not show a damped random walk (DRW) trend; (3) the mock DRW light curves present periodic-like trend on rare occasions in 10000 simulations; (4) the relativistic boost scenario is favored, since the relation between variability amplitude and power-law slope ratio is consistent with the theoretical prediction of this scenario, and a substantial parameter space exists between the inclination angles and the black hole mass; (5) the circumbinary disk model is insufficient to explain the spectral energy distribution of our target; (6) the significant radio flux density of WISE J0909+0002 is not detected from the VLA FIRST Survey, thus the radio jet precession scenario is ruled out. The Doppler boost scenario is, from our results, likely as a cause of the periodic variability; consequently the quasi-periodic oscillations in WISE J0909$+$0002 is possibly interpreted by a supermassive black hole binary. Additional observations to investigate the continuity of the periodic trend would bring new insights into mechanisms of the quasi-periodic oscillations and/or ELIRGs.