Testing period diagnostics on quasar periodic candidates in the era of large surveys

We assess the variability of four quasars previously identified as periodically variable quasar candidates. We analyze the extended light curves which now include Pan-STARRS1 (PS1), additional Zwicky Transient Facility (ZTF) observations together with archival Dark Energy Survey (DES) data that were not considered in the previous study, substantially increasing both the temporal baseline and sampling. Periodicity is re-evaluated using two independent period-search methods, enabling a direct comparison of the inferred periods across different data sets and temporal blocks, which are tested for the effects of cadence or stochastic (red-noise) nature. In addition, we investigate the structure function, phase-folded color{magnitude behavior, and timescale dependent color variability to assess whether the apparent periodic signals are accompanied by other coherent (chromatic) variability. This combined analysis presents a possible methodology for the assessment of future large multi-band time-domain surveys, which should aim to probe the robustness, persistence, and physical nature of the possible periodicity in faint quasars.