Optical QPOs with different periodicities in CSS and ZTF light curves of the quasar 4C 50.43

Abstract

Long-standing optical quasi-periodic oscillations (QPOs) with periodicity of hundreds to thousands of days have been accepted as indicators for central sub-pc binary black hole systems (BBHs) in broad line active galactic nuclei (BLAGN). However, there are so far no direct reports on whether such reported optical QPOs have their periodicities constant in different periods. Here, based on different methods applied to light curves of 4C 50.43 in different periods, optical QPOs with periodicity of 1124days was detected in the CSS V-band light curve, while a shorter periodicity of 513days was detected in the ZTF g/r band light curves. Despite the two periodicities near-harmonic 2:1 ratio, their absence of simultaneous detection in the lomb-scargle periodograms of the ZTF light curves suggests that they are unlikely to be harmonically related. Potential factors were considered to explain these two distinct periodicities, especially different temporal coverage, signal-to-noise ratio and time steps between the CSS and ZTF light curves, as well as the effects of red noises related to intrinsic AGN variability. Our analysis shows that red noises have strong influence on the different periodicities in 4C 50.43 supporting our previous simulations. The results in this manuscript strongly indicate that it should be cautioned for applications of determined optical QPOs in BLAGN having strong intrinsic AGN variability.

Figures (3)

• Figure 1: Light curves fitting result of the CSS-V band (left, red symbols) and the ZTF g/r band (right, blue and green symbols). The first row presents the direct sinusoidal fitting results: in top panel, the solid red lines and orange dashed lines show the best-fitting results and corresponding 5$\sigma$ confidence bands from a sinusoidal function plus a fifth-degree polynomial component. The dashed purple lines represent the polynomial components, and the solid purple line represent the determined sinusoidal components. The second row show the phase-folded fitting results: in top panel, the solid red lines and dashed orange lines show the best-fitting results and the corresponding 5$\sigma$ confidence bands. The bottom panels of each row display the corresponding residuals (light curves minus the best fitting results), with solid red line showing residuals=0.
• Figure 2: Top panel shows the GLS periodograms of light curves. Solid lines in red, blue, and green show the corresponding results from the CSS V-band and ZTF r/g-band light curves. The horizontal dashed lines in red indicate the corresponding 5$\sigma$ significance level (FAP = 3e-7). Middle and bottom panels show distributions of the periodicity determined by the bootstrap method from the CSS V-band light curve and the ZTF g/r-band light curves.
• Figure 3: Upper left panel shows distribution of the ratio of the periodicity $T$ to the measured periodicity detected by GLS method applied in the simulated light curves including sine component and intrinsic AGN variability. Upper right panel shows the GLS periodograms of five light curves randomly selected from the CSS V-band light curves, with time spans 2236days, 2475days, 2217days, 2208days, and 2466days (in orange, green, red, blue, and purple, respectively), similar to that of the ZTF g band. Lower left and right panels show periodicity ratios between each subset with varying time steps (lower left) and varying signal-to-noise ratios (lower right) and the original ZTF g band light curve.