Optical quasi-periodic oscillations of the quasar 4C 50.43 with different periodicities in CSS and ZTF light curves

Long-standing optical quasi-periodic oscillations (QPOs) with periodicities of hundreds to thousands of days have been accepted as indicators of central sub-parsec binary black hole systems in broad line active galactic nuclei (BLAGNs). However, there are so far no direct reports confirming whether such optical QPOs have constant periodicities. Here, based on different methods applied to the light curves of 4C 50.43 detected at different times, an optical QPO with a periodicity of 1124 days was detected in the CSS V-band light curve, while a shorter periodicity of 513 days was detected in the ZTF g- and r-band light curves. Despite the two periodicities near-harmonic 2:1 ratio, the fact that they were not simultaneously detected in the Lomb-Scargle periodograms of the ZTF light curves suggests that they are not harmonically related. We considered potential factors to explain these two distinct periodicities, notably different temporal coverages, signal-to-noise ratios, and time steps between the CSS and ZTF light curves, as well as the effects of red noises related to intrinsic active galactic nucleus variability. Our analysis shows that red noises have a strong influence on the periodicities in 4C 50.43, which is in line with our previous simulations. Our results strongly indicate that caution should be taken when applying optical QPOs in BLAGNs with strong intrinsic active galactic nucleus variability.