2025A&A...698A.265L

This manuscript investigates the possible existence of a binary supermassive black hole (BSMBH) system in the blue quasar SDSS J100438.8+151056 (= SDSS J1004+1510) at z = 0.219 based on the detection of robust optical quasi periodic oscillations (QPOs). We determine QPOs using multiple analysis methods applied to the Catalina Sky Survey (CSS) V-band, Zwicky Transient Facility (ZTF) g-band and r-band light curves, and additionally, combined with the characteristics of broad emission lines, we explore potential mechanisms for the QPOs, including jet and disk precession models. Two distinct periodicities, 1103±260 days and 243±29 days, are identified in the ZTF g-band and r-band light curves with a confidence level exceeding 5σ, through four different techniques. Meanwhile, the 1103±260 days periodicity is also clearly detected in the CSS V-band light curve. The optical periodicities suggest a BSMBH system candidate in SDSS J1004+1510, with an estimated total virial black hole (BH) mass of (1.13±0.14)×10⁸ M_⊙ and a space separation of 0.0053±0.0016 pc for the periodicity of 1103±260 days. The second periodicity, of 243±29 days, could be attributed to harmonic oscillations, considering (1103±260)/(243±29)∼4.54±0.47 with large scatters. However, if the periodicity of 243±29 days was from an independent QPO, a triple BH system candidate on a subparsec scale could probably be expected, with space separations of 0.00036±0.00004 pc within a close BSMBH system and of 0.0053±0.0016 pc between the BSMBH system and the third BH, after considering the similar BH mass of the third BH as the total mass of the central BSMBH. These findings strongly demonstrate that combined light curves from different sky survey projects can lead to more reliable QPO candidates being detected, and also indicate that higher-quality light curves could probably be helpful in finding potential QPOs with multiple periodicities, leading to rare detections of candidates for subparsec triple BH systems.